Condensed and Dried Milks

Module 13. Technology of dried milks

Lesson 33
TWO-STAGE DRYING

33.1 Introduction

Spray drying is relatively expensive with respect to energy and the capital outlay for driers is high. Hence, a better efficiency is required and in principle, it can be achieved by increasing the concentration factor of the milk before atomization as well as by applying a higher air inlet temperature. However, these measures can lead to heat damage of the product. Alternatively, the powder can be separated from the air before it is completely dry, while additional drying occurs outside the drying chamber. In this way, the outlet temperature of the air can be lower, allowing the inlet temperature of the air to be higher without increased heat damage occurring. Moreover, a larger quantity of concentrate can be dried per unit time.

33.2 Process

The powder may be discharged from the drying chamber after it has become so dry as to have lost its stickiness. The problem of stickiness is less than expected because of the concentration gradient formed in the powder particles. In the center of the particle, the water content is 24% for drying time of 0.8s. It is on average about 13% but only about 2% at the periphery. Presumably, these particles would still be slightly sticky because

(1) Stickiness i.e. the tendency to stick to the machinery considerably increases with temperature; and

(2) Upon removal from the drying air, the outside of the powder particles rapidly increases in water content due to internal exchange of water. Moreover, larger particles will be ‘wetter,’ hence, more sticky. But a powder with an average water content of about 8% can readily be discharged by means of cyclones.

The final drying is often achieved in a fluid bed drier . A layer of powder deposited on a perforated plate can in principle be fluidized by blowing air through the layer from below. In such a fluidized bed the powder layer is expanded, containing a high volume fraction of air; the mixture can flow, almost like a liquid, if the perforated plate is slightly tilted. The particles in the bed are in a constant erratic motion, which enhances drying rate. The conditions for fluidization are

(1) The particles size is from ~20 µ m to a few mm and their size distribution is not very wide; and

(2) The air flow is evenly distributed over the bed and has a suitable velocity, e.g., about 0.3 m/s for most spray-dried powders.

Generally, the size distribution of milk powder particles is too wide: if all particles are to be fluidized, including the largest ones, the air velocity must be so high that the smallest particles are blown out of the bed. To overcome this problem, the machine is made to vibrate, which allows fluidization at a lower air velocity. Such a fluid bed drier can then be attached to a spray drier by a flexible pipe.

In a spray drier the air inlet temperature is high; the holdup time of the powder is short, say, a few seconds. In a fluid bed drier the air inlet temperature is relatively low (e.g., 130°C), little air is consumed, and the residence time of the powder is much longer, i.e., several minutes. Because of this, a fluid bed drier is much more suitable for the final stages of drying. For example, in a comparison between traditional and two-stage drying as shown in Table 33.1, using the same spray drier, the same skim milk concentrate with 48% dry matter, dried to the same water content of 3.5%, may yield the following:

Table 33.1 Comparison between traditional and two-stage drying parameters

The efficiency of the heat expenditure thus is better (by 17%) and the capacity greater (by 57%); against this is the capital outlay for the fluid bed drier. The additional drying consumes only 5% of the heat. The quality of the powder (insolubility index) is certainly not poorer but, generally, better.

A fluid bed also offers additional opportunities such as,

1. It is quite simple to add a cooling section.

2. The bed can also be used for agglomerating purposes. The main incentive for agglomeration is that a fine powder poorly disperses in cold water.

3. Therefore, often an attempt is made to produce a coarse-grained powder. In the fluid bed the powder particles collide intensely with each other. As a result, they agglomerate if they are sufficiently sticky, i.e., have high enough water content at their periphery. Hence, agglomeration is enhanced by blowing steam into the powder (this is called rewetting, which is mostly applied when producing skim milk powder).

The air velocity in the fluid bed may be adjusted in such a way that the smallest powder particles (which have already become very dry and therefore show poor agglomeration) escape separation. The latter particles are fed back to the drying chamber, gain entrance to the atomized liquid, and become agglomerated with the drying droplets. This process is especially applied for whole milk powder.

Two-stage drying can also be effected in a modified spray drier chamber. In the bottom end of the chamber, a fluid bed is realized. It need not be vibrated as the smallest particles are blown toward the atomization region where they agglomerate with the drying drops. The air circulates in the vertical direction and is removed near the top end of the chamber. A great variety of configurations has been developed: for instance, a combination of the type depicted in (b) with a fluid bed attached as in (a) of Fig. 33.1.

Another method of two-stage drying is carried out in the filter mat drier . The first stage is conventional spray drying. Atomization is by nozzles, and the flows of air and drop spray are cocurrent. The partly dried drops fall on a moving perforated belt. The spent air is removed through the powder bed formed on the belt and is sent to cyclones. The fines removed by the latter are added to the second section of the machine, where it agglomerates with the powder on the belt. In the third section, hot air is blown through the bed for final drying. The dry material then reaches the fourth section, where it is cooled. The powder particles become strongly aggregated and form a porous cake, which falls from the belt in large lumps. These are gingerly ground and the resulting powder is packaged.

The filter mat drier allows a greater part of the water to be removed in the second stage because the powder can hit the belt when still being sticky. The latter also makes this type of drier suitable to handle very sticky materials such as cream powders.

33.3 Manufacture of Nonfat Dry Milk

The manufacture of nonfat dry milks is shown as a Flow chart in Fig. 33.2. For the manufacture of Non fat dry milk (NFDM)/ skim milk powder, the pasteurization can be less intense (at least phosphatase negative), according to the intended application. Homogenization is omitted, and the milk can be concentrated to somewhat higher solids content. Also no lecithinizing and gas flushing carried out. Sometimes vitamin preparations are added, especially vitamin A. This can be achieved by dry mixing afterward, or by emulsifying a concentrated solution of vitamin A in oil into a part of the skim milk.

The equipment for processing most dry milk products usually includes a separator, preheated, and/or high temperature short time pasteurizer with flow diversion valve, hot well, evaporator, preheater, filter in concentrate line, high pressure pump and other pumps, drier with milk dust collector, cooler, sifter, and packaging equipment. Many variations in equipment and methods are in use. In fact, no two plants are exactly alike in equipment and method.

In the design of equipment layout, a simple forward flow arrangement is best for quality and efficiency. The least equipment contact and the faster the forward movement of the product, the better will be the quality of the dry milk. Recycling may be detrimental to product quality and should be limited especially for low-heat nonfat dry milk. Elimination of recycling is more difficult for equipment that is used to dry several products compared to single product processing.