Types of spray drying installations

The capability of spray drying installations is determined by the type of chamber and by the type and combination of the components incorporated. Any modern spray drying installation must have the components as given in chapter 4. What makes the difference between various installations as to their ability to fulfil the qualitative and economical requirements is the type of after treatment system utilized and mutual compatibility of the components selected. From this point of view, spray drying installations can be divided into single-stage, two-stage, three-stage (like GEA Niro´s MSD™ plant) and special systems. Furthermore, depending again on the type of individual components, installations can be distinguished by their ability to produce either regular (non-agglomerated) powders or agglomerated products.

The flow sheets of the individual installations in this section are presented with the main components drawn only by symbols, which are self-explanatory. Obviously all installations can have additional components such as bag filters, heat recuperation etc.

5.1.

Single stage systems

These are the simplest types of installations. Any type of chamber described in chapter 4 and shown on the first two rows of Fig. 4.2., can be used for drying milk products, usually in combination with a pneumatic conveying system or external fluid bed. However, some designs operate without any form of such after treatment.

5.1.1.

Spray dryers without any after-treatment system

Spray drying chambers where powder leaves the chamber together with the drying air do not need any after-treatment system, if the powder can be bagged-off without cooling. These types of drying chambers represent standard equipment used in the early days of drying milk. This goes also for the box dryer which has a bag filter built into the drying chamber discharging the fines fraction back into the chamber. Space requirements are small, and building costs are low.

Generally, installations without any after treatment system are only suitable for nonagglomerated powders not requiring cooling (cooling is often necessary to avoid powder caking in bags). The flow sheet of such system is shown on Fig. 5.1. This system can also feature a chamber with flat bottom with either a rotating mechanical powder scraper or rotating suction arm (see Fig. 4.2.) to convey the powder into the exhaust duct.

**Şek.5.1. Spray dryer without any after treatment**

5.1.2.

Spray dryers with pneumatic conveying system

The aim of the pneumatic conveying system is to cool the powder while transporting the chamber fraction and cyclone fraction to a single discharge point.

This type first appeared in the 1950’s and dominated the milk powder industry up to the middle of the 1960’s. A typical flow-sheet is shown on Fig. 5.2. It is suitable for production of non-agglomerated powders. The fat content of the powder must not be higher than about 35%.

**Şek.5.2. Spray dryer with pneumatic conveying system**

5.1.3.

Spray dryers with cooling bed system

The primary reasons for introducing fluid beds into dairy drying installations were (a) the disadvantage of pneumatic conveying systems not being able to handle milk powders of high fat content, (b) the introduction of milk replacers with high fat contents for feeding calves and (c) producing in winter high fat powders which were standardized to normal fat content by dry mixing with skim milk powder from the peak season. It was also very convenient for the fines from main cyclone and fluid bed cyclone to be transported via the rotary valves below the cyclones by gravity or screw conveyor into the fluid bed. This plant configuration is today considered obsolete and not further discussed in this second edition.

It was however later found that the fluid bed had another advantage over pneumatic conveying. It not only collects and cools the high-fat powder, but it was observed that when using the same system for skim milk the powder was more coarse, better free flowing and more easy to reconstitute. This was because the fluid bed provides more gentle treatment than pneumatic conveying system by not breaking down the primary agglomerates. Furthermore it also had the ability to classify the powder i.e. to blow-off the finest size fractions. The possibility of manufacturing instant powder and the importance of agglomeration for instant properties was already known from the rewet process which was introduced at the beginning of the 1950’s.

Therefore the next logical step was to introduce the fines from the powder separators back to the wet zone of the drying chamber to support the agglomeration. This resulted in the successful development of the so-called cooling bed process for producing a sort of instant powders, even when the fluid bed was supplied with only cold air. This installation which can operate alternatively with high fat powders (introducing the fines back to the fluid bed) or agglomerated powders (with fines to the wet zone) is shown on Fig. 5.3.

The spray drying installations with cooling bed are still used for products which cannot be processed successfully by two stage drying such as high fat milk powders etc.

**Şek.5.3. Spray dryer with a VIBROFLUIDIZERTM as cooling bed**

5.2.

Two stage drying systems

The principles and advantages of two stage drying were described in chapter 3.

The second stage of drying can be conducted in either an external or an integrated fluid bed. Two stage drying installations can also be operated as single stage installations, if the product characteristics require single stage processing. In such case there is no heating of the fluid bed air.

5.2.1.

Spray dryers with fluid bed after-drying systems

Referring again to the development of cooling bed systems discussed in previous section, it was soon recognized that when operating the dryer with lower outlet temperature and producing powder of high moisture content, it was better agglomerated and consequently had better instant properties. Introducing heated air in the first section of the fluid bed removed the excess moisture while maintaining the instant properties. This process became known as straight through process and dominated production of instant milk powders especially instant whole milk powder from 1970 to about the middle of the 1980’s. It is still extensively in use in spite of further developments discussed below. Originally this system was provided with fines return system below the wheel (Fig. 4.19.). Today’s modern installations have the fines return from above the wheel (Fig. 4.20.). This system can operate also with pressure nozzles. The flow sheet is shown on Fig. 5.4. Obviously any type of chamber from which the powder leaves under gravity can be used for two stage drying with a fluid bed after drying system.

Soon after the development of the straightthrough process, which is characterized by introducing fines into the wet zone around the atomizer and by fluid bed after drying, it was found that the same installation operating in principle with the same conditions but with introduction of fines into the fluid bed, can produce also non-agglomerated powders while still utilizing the advantages of two stage drying.

**Şek.5.4. Spray dryer with a VIBROFLUIDIZERTM as after dryer/cooler and fines return**

Products leaving the fluid bed are still somewhat agglomerated. However with a simple blow line, transporting the powder to silos, which was anyhow necessary taking into account the yearly capacity growth, sufficient break-down of agglomerates occurred. The resulting powder, utilizing the benefits of two stage drying, was heavier than from a single stage process. The overall powder quality was also better and the overall energy consumption lower.

5.2.2.

TALL FORM DRYER™

The TALL FORM DRYER™ is a spray dryer with a tall slim drying chamber with top mounted nozzle assembly featuring a fines return capability. See Fig. 5.5. The resulting powder is discharged into a fluid bed for final drying and cooling, while the exhaust air is discharged through the enlarged lower cone section called a ’bustle’. This type of dryer is suited for both non-fat and fat-containing products, producing non-agglomerated and agglomerated freeflowing powders including baby food.

The special air outlet further means that there is nothing inside the drying chamber that might obstruct the air flow, such as ducts etc. where powder can stick. The atomization of the feed material is always done by high pressure nozzles, which form an integrated part of the air disperser. The air disperser is designed to assure a high velocity drying air stream necessary to obtain a final product with good re-solubility properties.

All process air is passed through cyclones and/or bag filters for separation of the entrained fines powder particles. These fines are then returned to the fluid bed for regular non-agglomerated powders and/ or the atomization device for production of agglomerated powders.

The advantage of the TALL FORM DRYER™ (TFD) is the downward drying air stream in the drying chamber, since this reduces the tendency of powder deposits on the chamber wall. The specially designed chamber cone with a “bustle” air outlet reduces the amount of fines powder particles carried along with the air into the fines collector.

5.2.3.

Spray dryers with Integrated Fluid Bed

The background and principles of the integrated fluid bed are described in chapter 3. It was introduced in 1980 and became very soon a dominating design of spray dryer not only in the milk powder industry, for which it was originally developed, but also in other industries (used for chemicals, pharmaceuticals, coffee, sorbitol etc.).

The integrated fluid bed is non-vibrating (so-called static) and can either be circular (used in MSD™ chambers) or annular (used in COMPACT DRYER™ chambers). Both chambers are shown on Fig. 4.2. Typical ratios of primary drying air through air disperser to secondary drying air (static fluid bed) are in the range 3-4 to 1.

Both of these chamber types operate as two stage drying systems but only for the production of non-agglomerated powders. This is because the integrated fluid bed powder fraction is collected together with the cyclone fraction and during pneumatic conveying, agglomerates are broken down.

The combination of integrated fluid bed dryer with pneumatic conveying system is applied in the milk powder industry in connection with GEA Niro COMPACT DRYER™ chambers for the production of non-agglomerated powders and the installation is called CDP (Compact Dryer Pneumatic) – see Fig. 5.6. The combination of the chamber of a Multi Stage Dryer with pneumatic conveying system is used extensively in other industries and known as a FSD™ (Fluidized Spray Dryer). Fig. 5.7. shows a GEA Niro FSD™ dryer with pneumatic cooling system.

**Şek.5.8. Fluidized spray dryer with integrated fluid bed and pneumatic conveying system**

5.3.

Three stage drying systems

These installations with a static fluid bed as a second stage dryer in combination with external vibrating fluid bed as a third stage dryer appeared for the first time at the beginning of the1980’s and were called COMPACT DRYER™ (GEA Niro) type CDI (I for Instantization) and . They dominate today the milk powder industry.

Three stage systems combine all the advantages of extended two stage drying using spray drying as the primary stage, fluid bed drying on a static fluid bed as second drying stage, external vibrating fluid bed as the third. The final drying stage terminates with cooling. Evaporation conducted in each stage can be optimised to achieve both gentle drying conditions and good thermal economy.

**Şek.5.9. Compact dryer and multi stage dryer**

5.3.1.

COMPACT DRYER™ type CDI (GEA Niro)

The COMPACT DRYER™, as shown on Fig. 5.8. is suitable for producing both nonagglomerated and agglomerated powders of practically any kind of dried dairy product. It can cope successfully also with whey powders, fat-filled milk and whey products as well as caseinates, both non-agglomerated and agglomerated. It has a limitation as to fat content, which is about 50% fat in total solids. The powder quality and appearance is comparable with products from two stage drying systems, but they have considerably better flowability and the process is more economical.

**Şek.5.10. COMPACT DRYER™ with VIBRO-FLUIDIZERTM and fines return**

Installations with capacities higher than approximately 1500 kg/h powder production have better performance than smaller units in all respects and especially powder quality.

5.3.2.

Multi Stage Dryer MSD™ type

The Multi Stage Dryer, MSD™ Fig. 5.11. in comparison with the COMPACT DRYER™ can process an even wider range of products and handle even higher fat contents. The structure and appearance of MSD™-dried powders differ from the products obtained in two stage drying installations and that from a COMPACT DRYER™. In fact it is different from all previously produced powders. The main characteristics of MSD™-powders relate to very good agglomeration and mechanical stability, low fractions of particles of size below 125μm and superb flowability. The mean particle size of, for instance, agglomerated whole milk powder may be adjusted between 180-300μm but the most typical and best functional properties are obtained at 250μm with bulk densities in the range of 420-480 kg/m3. MSD™-type spray dryers do not need any fines return system to obtain such agglomeration, but may be supplemented with such a system which then produces skim milk powder of mean particle size 500-1000μm and bulk density as low as 300 kg/m3. It is also suitable for high fat powders up to 80% fat in total solids.

However, high bulk density powders are difficult to produce in the MSDTM-type plant due to the spontaneous secondary agglomeration taking place in the atomization area as a result of the high drying air inlet velocity. By adjusting the operating conditions (higher exhaust air temperatures = lower capacity) it is possible to produce whole milk powder and skim milk powder having bulk densities up to 580-630 kg/m³ 650-720 kg/m³ respectively after milling and a blow line conveying system to powder silos.

5.3.3.

Spray drying plant with Integrated Filters and Fluid Beds - IFD™

The Integrated Filter Dryer design, see Fig.5.12., is based on proven spray dryer unit operations from the COMPACT DRYER™ and the MSD™ dryer and the development of the SANICIP™ bag filter such as:

**Şek.5.12. Integrated filter dryer IFD™ with integrated filters and fluid beds**

Feed system with concentrate preheating, filtration, homogenization, and highpressure pumps. All equipment as used in all other types of spray dryers
Atomization using pressure nozzle atomization
Drying air filtration, heating, and distribution using an air disperser suitable for vertical air streams
Drying chamber designed to ensure hygienic operation conditions and to maintain lowest possible heat loss by means of e.g. dismountable insulation panels featuring airfilled sandwich panels, see chapter 4
Integrated fluid bed designed as a combined back-mix bed for the drying and a plug-flow bed for the final drying and cooling. Between the back-mix bed and the surrounding plug-flow bed there is an air gap to avoid heat transmission
The dryer exhaust air system is new and though the idea is revolutionary, it is still based on the same principles as applied in GEA Niro’s SANICIP™ CIP-able Bag Filter. The fines collection system operates with particulate filters integrated in the drying chamber. The filter bags are supported on stainless steel cages mounted in the ceiling around the circumference of the drying chamber. These filter elements operate with blow-back air cleaning systems and CIP operation similar to those used in the SANICIP™. See chapter 4
The advantage of this dryer is the reduced building height/volume as there are no external fluid bed(s) for after drying and cooling and no external bag filter(s). Further a full CIP turnaround time - including also the dry out time - is only 6-8 hours. As the pressure drops over the air disperser and exhaust system is low, it results in low energy consumption and a low noise level.

5.3.4.

Multi Stage Dryer MSD™-PF

Based on the knowledge from operating the Multi Stage Dryer MSD™ and the Integrated Filter Dryer IFD™, a new dryer design was evolved. See Fig. 5.13. In a traditional MSD™ dryer with an external bag filter, the external fluid bed, the VIBRO- FLUIDIZER™ has been replaced by a circular plug flow fluid bed similar to that used in the Integrated Filter Dryer IFD™. The advantage of this design is a reduced building height.

5.3.5.

FILTERMAT™ (FMD) integrated belt dryer

This type of dryer differs substantially from the installations discussed so far. It has all the drying stages conducted in separate compartments of one box-type chamber.

The air flow in the spray drying section and also the after-drying, conditioning and cooling sections is streamline downward. The bottom of the chamber is formed by a conveyor belt, which is a mesh made of polyester material. Atomization is by multi-nozzles type and each nozzle is placed in a separate air inlet. Moist powder lands on the belt forming a layer through which the drying air is passing downwards (see Fig. 5.14.). Thus the moist powder layer acts as a filter resulting in negligible amounts of fines passing with the exhaust air through the belt and to the cyclone. This effect is the same in all sections.

In chapter 3 limitations were mentioned as to powder moisture levels from the primary stage in two stage drying. Milk powder of high moisture cannot withstand any mechanical handling, and this includes just the rolling down of the powder over the cone of conventional chamber design. This is especially the case with highly thermoplastic and sticky powders of high carbohydrate and fat content. The extended two stage drying, as conducted in spray dryers with integrated fluid beds, has shifted these limits, although there is some mechanical treatment and contact with the walls. The FILTERMAT™ Dryer (having just a horizontal base formed by a porous belt on which the powder can settle and be after-dried) creates no mechanical treatment and thus is suitable for highly thermoplastic and sticky materials such as whey products, fat filled whey powders and especially for whey and milk products with hydrolysed lactose, fruit juices and tomatoes etc. This type of dryer is not particularly suited for high protein containing products like skim milk and WPC.

5.4.

Spray dryer with after-crystallization belt

This type of dryer is in principle a two stage drying installation but has rather special and many original features that require separate description. The flow sheet is shown on Fig. 5.16. It is a single duty spray dryer for non-caking whey and permeate powder. The production technology involved is described in chapter 8. However, it provides a most economical production method being able to operate at low air outlet temperature and high feed concentration securing at the same time a high quality non-caking product. Special features of this dryer, which can be seen on the flow sheet, is the after crystallization belt ensuring that the moist powder (8-10%) has a residence time of 6-8 minutes. This means that the remaining amorphous lactose has time to crystallize as there is enough “mobile water” and sufficient time prior to entering the fluid bed for final drying and cooling.

Typical operating conditions are: inlet temperature 150-160°C, outlet temperature 55-58°C. The feed should be pre-crystallized and have a concentration of 55-65%. The resulting powder can be termed 100% non-caking and non hygroscopic as all the lactose has been crystallized. And that means that the powder can be exported and used also in countries where high air humidity is prevailing, and that without the powder will start to cake and lump.

**Şek.5.16. TALL FORM DRYER™ (TFD) with after crystallization belt**

5.5.

TIXOTHERM™

Whey and permeate from ultra-filtration of whey and milk is considered low-price byproducts, if processed into a powder or expensive if disposed of into sewage plants. With the increasing cheese production there will be more and more whey and/or permeate available. A new process – the TIXOTHERM™ process has been developed. It will convert this by-product into a first-class 100% non-caking powder product using less energy and a reduced building volume means lower total investment.

Pre-crystallized whey and permeate concentrates are very thixotropic, i.e. if the concentrate is not agitated continuously in the crystallization tank in the traditional process, it will solidify, and the higher total solids in the concentrate, the more solid becomes the concentrate. This phenomenon is used advantageously in the TIXOTHERM™ process. See Fig. 5.17.

The TIXOTHERM™ process is a four-step process, where the whey or permeate is evaporated in a falling film evaporator to 60 % TS. From the evaporator the concentrate is pumped to a vertical agitated film high concentrator. Here the solids is increased to 85% TS. The high shear rate in the agitated film, however, keeps the viscosity relatively low. The combination of high solids and low temperature as a result of the evaporation initiates the lactose crystallization, and many nuclei are formed.

In the subsequent process – the mixing crystallizer – a phase-conversion of the product takes place enhanced by addition of fines particles/recycled cold product from the subsequent fluid bed drying/cooling. This encourages rich lactose crystallization.

At the discharge of the mixing crystallizer, the product is now like a semi-solid mass with a friable texture ideal for the following fluid bed drying and cooling. In this last step of the process the remaining moisture is evaporated in an agitated back-mix fluid bed followed by final drying and cooling in a plug flow fluid bed. The drying air for the bag mix and plug flow fluid bed is exhausted through a bag filter, from where the collected fines particles are returned back to the mixing crystallizer.

The final powder has almost 100% of the lactose crystallized, i.e. the total moisture content is 5% or more. The powder is non-hygroscopic and non-caking. Energy consumption is reduced by more than 30%, and building requirements are reduced by more than 50 %. No crystallization tanks are needed, and no spray drying plant is involved to produce whey or permeate powder.

The capacity of a given TIXOTHERM™ plant is a function of the content of protein, galactose and lactic acid in the product. That is why a TIXOTHERM™ plant for whey/permeate from mozzarella cheese has a lower capacity than for instance whey/permeate from Swiss cheese. The disadvantage of this plant is that it can only be used for whey and permeate.

5.6.

Choosing a spray drying installation

The decision as to the choice of a spray drying installation for a powder plant - regardless whether it is for a new green field site or additional equipment for an old plant - requires an extensive preliminary preparation and study. First of all it is necessary to make an analysis, which must be based on market conditions, the range of products which will be produced and their quality specification. Secondly the installation capacity must be decided, based on prognosis of milk availability in the area in question. The decision will influence the performance and economy of the whole powder plant for the next many years.

The easiest task, which is giving the possibility to choose a plant with optimum performance both as to product quality and economy, is when it has to produce one product only. However, a single duty installation is very seldom in question. A choice of an installation for two or more duties might often be a compromise with a consequence that some of the duties will not be fulfilled to the optimum. The capabilities of various spray drying installations as to various products and qualities are very different and there is no installation which can cover the whole range to the optimum. Besides one has to be aware that nowadays the dairy drying installations are becoming more and more food drying plants which have to process not only pure dairy products, but also milk based products with other food additives and even non-milk compositions.

The main aspect to take into account is product quality, length of continuous production (20 hours or more) and production economy. The general rule for installations with a choice between rotary wheel and pressure nozzle atomization is that pressure nozzles can comply better with high bulk density powder specifications while a rotary wheel gives the possibility of handling feeds of higher total solids content. Similarly, with a choice between a conventional chamber and a TALL FORM DRYER™, the latter allows longer periods of operation, and tendencies for sticky powders, i.e. high carbohydrate and/or high fat products to deposit on the walls are smaller.

Generally it can be stated that the FMD™ is first preference for sticky powders, the MSD™ leads for agglomerated instant powders including baby food, and the TALL FORM DRYER™ and the COMPACT DRYER™ are ideal for fat-filled powders and baby food.

Okumaya devam etmek için lütfen kaydolun

Handbook of Milk Powder Manufacture

Hazırlayan Dr. Ing. Jan Pisecky

Handbook of Milk Powder Manufacture cover

The Handbook of Milk Powder Manufacture is a valuable reference book for dairy processing engineers wanting to take a deeper look at the complex world of milk powder processing.

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