Spray Driers

i. Spray Drying Process

Spray drying is a gentle drying method. The material to be dried is suspended in the air, and drying time is very short. Air inlet temperature up to 215°C are used for  drying of milk, but due to evaporation, the temperature drops immediately in drying chamber, and the milk solids do not reach a temperature approaching the air inlet temperature. The temperature in the drying chamber is equal to the air outlet temperature, which is approximately 95°C (single stage) and lower (multiple-stage drying). The product temperature will be 20-30°C below the air outlet temperature.

Spray drying process takes place in three major stages:

•  Atomization: Dispersion of the hot liquid into a fog like mist.
•  Mixing of the hot fog like mist into a stream of hot air which quickly evaporates the water.
•  Separation and collection of powder from the drying air

ii. Classification of Spray Driers

i) Method of Atomization

a) Rotary wheel atomizers (Centrifugal disc) b) Pressure nozzle single-fluid atomizers (pressure spray nozzle) c) Pneumatic two fluid nozzles (compressed air)

ii) Method of heating air

a) Direct (gas or fuel oil) b) Indirect (Utilizing heat exchanger plate or coils)

iii) Method of furnishing heat

a) Steam b) Gas c) Fuel oil d) Electricity

iv) Position of drying chamber

a) Vertical b) Horizontal

v) Number of drying chambers

a) One (Main only) b) Two (Main and subsidiary)

vi) Direction of air-flow in relation to product flow

a) Counter current b) Parallel (concurrent) flow c) Right angled

vii) Pressure in drier

a) Atmospheric (usually a very slight pressure) b) Vacuum

viii) Method of separation of powder from air

a) Cyclone separators b) Multi-Cyclone separators c) bag filters d) Liquid dust collector e) Electrical dust collector f) Electro-static precipitators

ix) Treatment and movement of air

a) Recirculation of air ii) Dehydration of air iii) Conventional- atmospheric air received and exhausted after use

x) Method of removal of powder from drying chamber

a) Conveyer b) Vibrator c) Sweep conveyor d) Air conveyed to the  cyclone

xi) Method of heat transfer

a) Convection b) Radiation

xii) Kind of atmosphere in drying chamber

a) Nitrogen b) Air c) Other gas, usually inert

xiii) Position of fan providing

a)      Pressure in chamber b) Suction in chamber

xiv) Direction of air-flow in chamber

    

    a) Updraft b) Downdraft c) Horizontal d) Mixed

xv) Space of drying chamber

    a) Silo or cylindrical b) Box like c) Square cross section d) Tear drop

xvi) Product being dried

    a) Milk b) Other milk products c) Other food products d) Detergents

iii. Atomization

Atomization is the most important operation in the spray drying process. The atomizer produces fine particles, uniform in size, having a diameter of 20-150 çm (most of which are in the range of 50-80 çm) with a high surface area to mass ratio, thus enabling quick heat transfer with a high evaporation rate. The type of atomizer not only determines the energy required to form the spray, but also the size and size distribution of the drops, on which the final particle size depends. The chamber design is also influenced by the choice of the atomizer. The drop size establishes the heat transfer surface available and thus the drying rate. Three general types of atomizers are available. The most commonly used are the centrifugal (rotary) atomizer and the pressure (nozzle) atomizer. Pneumatic two fluid nozzles are used only rarely in very special applications.

A centrifugal atomizer is designed as a disk with vanes. The milk is transported to the atomizer under normal pressure and sprayed into fine droplets by centrifugal atomizer at rotating speeds of 10000-20000 rpm. A centrifugal atomizer is advantageous in drying viscous materials and suspensions.

A pressure nozzle, sometimes called a single fluid nozzle, creates spray as the liquid passes through an orifice of several millimeters in diameter under pressure within the usual range of 5-7 MPa. The liquid enters the nozzle core tangentially and leaves the orifice in the form of a hollow cone with an angle that varies from 40° to 140°.When larger feed rate is to be processed, several nozzles are used in the drying chamber. Owing to their smaller spray angles, the drying chamber can be narrower and taller. With this type of nozzle, it is generally possible to produce the droplets within a narrow range of diameters, and the dried particles are usually hollow spheres.Pressures nozzles are not suitable for highly concentrated suspensions and abrasive materials because of their tendency to clog and erode the nozzle orifice.Pneumatic nozzles are also known as two-fluid nozzles since they use compressed air or steam to atomize the fluid. In this case, the feed is mixed with the air outside the body of the nozzle. The spray angle ranges from 20° to 60° and depends on the nozzle design. Approximately 0.5 m3 of compressed air is needed to atomize 1 kg of fluid. The capacity of a single nozzle usually does not exceed 1000 kg/h of feed.Sprays of less viscous feeds are characterized by low mean droplet sizes and a high degree of homogeneity. With highly viscous feeds, larger mean droplet sizes are produced but homogeneity is not as high. Pneumatic nozzles are very flexible and produce small or large droplets according to the air-liquid ratio.

iv. Movement of Air

The ambient air is filtered, heated to 150-300oC and introduced into the drying chamber at a velocity of up to 50 m/sec. Air cleaning is usually performed by dry filters that are cleanable or disposable. Presently in use are filter constructions,where material that serves as a filter is supplied automatically from one roll and collected on another roll after use. Because of considerable air-flow, filter regeneration is constantly carried out using cleanable filters.

      a) Method of heating air: Air can be heated by mixing with combustible gases in a direct   gas fired heater, where burning products (of gas or oil fuel), together with hot air, enter the chamber. Although this method of air heating has high heat efficiency (100%) and low investment and maintenance costs, it is not commonly used in the dairy industry. The main problem is the contamination of milk powder with nitrogen oxides that are contained in combustion gases.Coming into contact with amines, they could produce nitrosamines, which are known to be carcinogenic. The indirect way of air heating is understood as heating by steam in a tubular or plate heat exchanger (radiator), liquid face heating, or indirect oil or gas heating.

b) Direction of air-flow in relation to product flow: The air stream moves either through the spray drier in the same direction as the milk (concurrent flow), in the opposite direction (counter current flow), or under an angle (mixed flow). Concurrent flow mix pattern is used where low product temperature is needed. Both horizontal and vertical chambers usually have concurrent flow.The essential advantage of counter current flow is that the hottest (incoming) air comes into contact with the already partially dry product, and this enhances heat and mass transfer, reducing energy consumption. However, the resulting milk powder is heated to a higher degree during the last stage of drying, whe casein, at high concentration, becomes especially susceptible to denaturation.Mixed flow with integrated fluid bed pattern is commonly used for producing agglomerated powders. Yet another type of mixed flow pattern (fountain type) is for coarse sprays in small chambers for non heat-sensitive products. Countercurrent flow and mixing pattern is used for products, which withstand high temperatures, have coarse particles and for high bulk density powders. In spite of the inferior heat economy, concurrent flow is predominantly used in the dairy industry because it enhances product solubility.

c) Method of separation of powder from air: Dry product is taken away immediately after drying, preventing further contact between powder and hot air. Powder can be separated from the air inside or outside of the drying chamber.Regardless of the way it is separated, additional special equipment is needed.In drying chamber with conical bottom, a greater portion of the powder is separated internally by gravity and is discharged. Only fines remain suspended in the air. In chambers with flat bottom, all of the powder has to be separated from the drying air. In both cases, cyclone separators are the most commonly used separation systems, when low cost, efficiency and cleanliness are the criteria. The principle of cyclone separation is based on centrifugal force; the air enters the cyclone with high velocity through the entering duct, the diameter of which is approximately four times smaller than the diameter of the cyclone cylinder. The air velocity decreases, allowing powder to fall by gravity to the bottom of the cyclone, where it is continuously removed. Today, there is in application a system of several cyclones with large diameter and one cyclone with small diameter at the end, which serves to separate fines For a medium cost operation with high efficiency and high running cost, bag filters are used for powder separation. When dealing with separation of large air volumes, electro-static precipitators are used. A combination of cyclone and wet scrubber is used for better product and fines recovery.

v. Basic Drying Installations

a) Single Stage drying: Single-stage drying is the simplest installation for making spray dried powder. It works in a single-stage drying principle, which means that removal of all moisture from the feed to the required final moisture of the powder takes place in the spray-drying chamber itself. The subsequent pneumatic conveying system serves only to collect the powder leaving the chamber.

b) Two Stage Drying: In the middle of the 1970s a two stage drying system with external vibrating fluid bed was developed and in 1980, stationary fluid bed integrated in the drying chamber was invented. Although both processes may produce agglomerated (instantized) as well as non-agglomerated powders, the dominating ones are those with the characteristics of instant powders.

The two stage drying consists of spray drying as the first stage and fluid bed drying as the second stage. The moisture content of the powder leaving the chamber is 2-3 per cent higher than the final moisture content. In the second stage the powder is discharged into the vibrating fluid bed dryer and cooler. The fluid bed consists of a stainless steel perforated plate through which air is blown upwards. The powder is ‘fluidised’ by the air, and high turbulence occurs which provides excellent conditions for heat transfer between air and powder.In the first section, the powder is dried to its final moisture content by air at 100-120°C, and in the second section the powder is cooled by air at about 10-15°C.

c) Three Stage Drying: Three-stage drying has been introduced in order to further improve the thermal efficiency of the drying process by transferring a greater portion of the evaporation from the first stage to the second and third stages. In three-stage drying the second stage is a fluid bed at the outlet of the drying chamber. This fluid bed is static (non vibrating), and the semi-dry powder drops over an adjustable weir into an external fluid bed for final drying and cooling. A low outlet temperature from the drying chamber can be used, as the powder is delivered from the first drying stage to the integrated fluid bed at high moisture content.

d) Integrated Belt Spray Drying System (Filtermat Dryers): In this, the drying chamber is rectangular with a number of vertical pressure nozzles in ceiling. As the height of the chamber is insufficient for drying to the final moisture content, the moist product builds up into a mat on a porous moving belt. The drying air goes through this layer of powder, which acts as a filter for the exhaust air. The moist product is moved on the belt into following sections for further drying and cooling. Compared with a fluid bed, the air moves in the opposite direction in the Filtermat dryer. This results in a compact cake of the product, which breaks off at the end of the moving belt. Milling of the product is necessary to get powder.This type of drier is required for some dairy products, foodstuffs and food ingredients that are difficult to spray dry due to their carbohydrate, fat or protein contents. Continuous powder production of this type of product in conventional one or two stage spray dyers is often associated with product quality deterioration, deposit losses and frequent cleaning procedures, and is, therefore, not acceptable. Successful drying of sticky, hygroscopic, thermoplastic and slowly crystallizing products into free-flowing agglomerated powders requires powder temperatures to be maintained at much lower levels than those possible in conventional spray dryer layouts. Completion of drying under these conditions also requires the powder to be held within the dryer for much longer times too.A specially designed integrated belt spray dryer fulfills all these criteria. Its design combines a co-current nozzle tower dryer with a built-in conveyor belt.The transport time while powder moves with the belt is several minutes offering sufficient time to complete powder drying, agglomeration and cooling while maintaining the required powder temperature.