Condensed and Dried Milks

Module 13. Technology of dried milks

Lesson 29
SPRAY DRYING

29.1 Introduction

The spray dryer design and technology provide the powder producer with the ability to control the powder form through the spray drying process. The spray dryers are designed to produce properties such as particle size, bulk density, moisture content, solubility, dispersibility etc. exactly as desired by the user. The products are applicable to all pumpable products, whether heat or non-heat sensitive.

29.2 Spray Drier

Fig. 29.1 gives one, considerably oversimplified, example of a drier layout. Actually a wide range of configurations is applied, according to type of raw material used, product specifications, and local possibilities (e.g. resources). Naturally, minimization of operating expenses is desired; this is not simple when the drier is to be used for a range of different products.

As the atomized product is introduced into the drying chamber, heated air is forced through the chamber. The air furnishes heat for the evaporation of the moisture and the air is a carrier for moisture to be removed from the drier. The air may be forced through the drier by either a pressure or suction system. The product and air must be separated after drying. The product is then cooled and packaged. Controls maintain the proper adjustment of the variables involved in drying. Additional operations may be utilized to provide a product that will dissolve rapidly. The capacity of commercial driers may vary from 180 to 4000 kg per hr and higher of dried product. Proper management of a drier operation is important to provide good quality solid products from good quality liquid products.

29.3 Objectives

* To remove the moisture

* To reduce the cost of transportation

* To improve storage of the product, and

* To provide a product fit for many food manufacturing operations.

29.4 Essential Processes

The use of the spray drier has increased in recent years and it is the most important method of drying liquids to solids for milk and milk products. The spray drier is flexible and can be used for many different food products. The system, incorporating various components of equipment, is complicated, even though the principle of operation is simple. There are several variants, but the following are essential process steps that are always involved:

29.4.1 Heating of the air

Generally heat exchangers are used. The classical medium is steam, but a pressure over 9 bars, hence a temperature over 175°C, is hard to reach. Consequently, hot gas is now often used, obtained by burning natural gas, predominantly methane. Another possibility is hot oil. Direct electric heating of the air can also be applied. Direct burning of natural gas in the drying air is very economical, but is undesirable (and generally illegal) as it causes some contamination of the powder with nitrogen oxides. The air is heated to ~ 200°C and leaves the drier at ~100°C.

29.4.2 Atomizing the concentrate in the air

A spinning disk is used for atomization of the liquid. The drops leave the disc in a radial and horizontal direction. The drying chamber must be wide to prevent droplets reaching, and thereby fouling the wall of the chamber. Currently, spraying nozzles are more commonly applied. Except for very small driers, several nozzles, arranged in one or more clusters, are installed. The drops leave the nozzles in a roughly downward direction. The air inlet is generally in the center of a cluster, also directed downward. The drying chamber can have a smaller diameter and often has a greater height, as compared to driers with disk atomization. The shape of the chamber is designed in such a way that the mixing of air and droplets and the course of drying of the droplets are (presumed to be) optimal. This produces such small droplets that they will dry very quickly, with either a spinning disk or a pressure nozzle.

29.4.3 Mixing hot air and atomized liquid

Drying occurs correspondingly. Air and liquid usually enter the drying chamber co-currently and are mixed so intensely that the air cools very rapidly. Consequently, the larger part of the drying process occurs at temperatures not much over the outlet temperature. The air inlet is generally tangentially, causing a spiral-like downward motion of the drying air.

29.4.4 Separating powder and consumed drying air

This is primarily achieved with cyclones, arranged in various ways. In a drier as seen from above, the air strongly rotates and the lower conical part of the drying chamber acts as a cyclone, and most of the powder is discharged below. The powder often is cooled before packaging. So-called fines, i.e., the smallest powder particles present, are returned to the drier, often close to the region of atomization. The air has to be cleaned before it is returned to the atmosphere. Commonly bag-shaped cloth filters are used. Another option is wet washing: the air stream is led through a falling spray of water, which is recirculated. The outlet air is still hot and part of the heat can be transferred to the inlet air in a heat exchanger.

29.4.5 Aggregation of powder particles

This can be achieved by nozzle spraying when the nozzles are arranged in such a way that the sprays overlap. Another method is to return the fines in a region where the drops are still fairly liquid. A third way is rewetting in a fluid bed. In all these situations, sticky powder particles are caused to collide with each other.

29.4.6 Second-stage drying

It can be achieved in various configurations. In the drying of a liquid, several stages can be distinguished, e.g. a stage in which the liquid turns into a more or less solid mass and a stage in which the solid mass obtained decreases further in water content (final drying). In milk products, a solid-like material is obtained at a water content near 8% (the product obtained is no longer sticky and appears to be dry), whereas a powder with, say, 3% water is desired. Traditionally, one process step is included in both drying stages. In spray drying, advantage is often taken of separating the final drying, which is generally achieved in a fluid bed , apart from the main process.

29.5 Drying Functions

The spray drier utilizes a product which is first condensed in a vacuum pan or an evaporator. The product is then atomized inside a drying chamber of a drier. The drying functions include:

* Moving the air

* Cleaning the air

* Heating the air

* Atomizing the liquid

* Mixing the liquid in the hot air

* Removing the dry material from the air

* Additional drying of the product

* Cooling the product, and

* Pulverizing and Sizing the product

29.5.1 Classification of spray dryers

The classification of spray dryers according to their different arrangements and types are shown in Table 29.1.

Table 29.1 Classification of spray dryers

29.5.2 Characteristics of spray driers

The acceptance of the spray drier is attributable to its favorable characteristics, particularly for a good product obtained and the economics of operation. These characteristics are:

(1) Continuous operation

(2) Little labour required to operate

(3) Handle many different products

(4) Heat contact to product is short during drying and removal (quality is more likely to be preserved as overheating is less likely)

(5) Thermal efficiency is low compared to other types of driers

(6) Operation dependent on large surface area of product which is obtained by different methods of atomizing

(7) Separation of air and product after drying is important

(8) Unit is easy to clean if operating properly

(9) Economy of moisture removal improved by condensing product before drying

(10) Product properties and quality may be effectively controlled

Normally, product does not contact drier surfaces until dried, (However, sticking of the dried or semi- dried material to the surface of the drier is a problem if the heat source fails and the product continues to enter the drier)