Lesson 32. SEPARATION OF AIR AND POWDER

Module 13. Technology of dried milks

Lesson 32
SEPARATION OF AIR AND POWDER

32.1 Introduction

As the product is dried, it is necessary to separate the dried product from the air. Without special design features, the product will be carried by the moist air from the drier. It is necessary to remove the particles

(a) To get a maximum yield from the dryer by saving all powder product and

(b) To avoid air pollution surrounding the drying plant.

32.2 Location

The powder may be separated from the air primarily either

(a) Inside the drier (internal) or

(b) Outside the drier (external).

In both cases, it is necessary to use an additional device or component outside the drier to remove the fine or small particles which will not normally settle out in the drier. External separation devices are used on all driers.

32.3 Methods

The products which are separated from the air in the drier can be removed from the drier by:

(1) An air brush, by which air from outside the drier either at room temperature or conditioned to a lower temperature is used to direct a jet of air to move the product from the bottom of the drier,

(2) A rake or broom which is pulled across the bottom of the drier,

(3) A conveyor - flight or auger type and

(4) A gravity system.

Vibrators are often attached to the sides of the drier to prevent; or reduce sticking of the powder and to move the product rapidly from the drier.

32.4 Variables

The variables affecting separation of powder from the air are

1. Particle size

2. Concentration

3. Nature of the material, and

4. Quantity of the product

In a drier, where most of the powder is removed internally, the quantity of air handled by the separation system will be the same as for external separation, but the quantity of product will be much less. The product nature or characteristics such as fat, moisture, cohesion, and friction greatly affect the efficiency of separation or collection. The efficiency of collection is designated as the ratio of the output divided by input times 100% with the product at the same moisture content at the two locations. Some manufacturers rate the efficiency of collection or separation on the basis of kgs of dry matter input. If all of a product at -three per cent moisture is collected, an efficiency of 103 % would be claimed using this method of rating.

32.5 Types of Separators

32.5.1 Cyclone separators

Cyclone or Multi Cyclone is a type of interstitial separator and is most commonly used for removing the dry product from the air. Air at a high velocity moves into a cylinder or cone which has a much larger cross-section than the entering duct. The velocity of the air is decreased in the cone, thus permitting the settling of the solids. The velocity of the air decreases near the wall of the cylinder or cone and the product falls by gravity and is removed from the bottom. Cyclones may be used for storage of product before packaging, to provide for a more efficient packaging operation.

Centrifugal Force: The centrifugal force acting on the particle for removing the product from the air is

formula

The centrifugal force on the product is exerted toward the edge of the cyclone. The weight of the particle is effective vertically in the direction of the outlet.

Arrangement: Cyclones can be used individually or in combination, to provide multi cyclone units.

Efficiency: The efficiency of separation with a cyclone unit is based on product, cyclone design, and on the size of particle to be removed, but losses range from 0.5 to 3 and average one per cent. The cyclone is normally used for separation of material between 5 and 200µ. As the size of particle decreases, the efficiency of the cyclone also decreases. A properly designed cyclone will remove 99% of the solids larger than 30µ, 98% of material larger than 20µ, 90% of material larger than 10µ, but only 50% of the material smaller than 5µ.

32.5.2 Cloth collectors or bag filters

They have been used for many years. The material may be cotton, wools or plastic. Approximately 0.031 – 0.046 m2 of bag or cloth surface per 0.028 m3 per minute of airflow is provided for external separator. Losses generally range from 0.2 to 0.5%.

32.5.3 Wet scrubber or liquid device

They may be used where the fine particles passed through other separation devices are removed from the exhaust air and returned to the incoming product for redrying. The wet scrubber provides for a high recovery of product and heat from the exhaust air. The disadvantages are the possibility of product deterioration and contamination.

32.5.4 Electrostatic, sonic or ultrasonic, electrical, and packed beds of granular or fibrous materials

These types of separation now also used in the milk industry to separate fine solids from the air stream.

32.6 Explosion

Explosion caused by concentration of organic materials in the atmosphere depends upon the increase in temperature of the particles to the ignition point. The possibility of explosion usually is designated on the basis of concentration of the product in the air. It is generally considered that a concentration of organic dust of 178.5 g/m3 or less is safe from the stand point of explosion. Moreover, the powder may catch fire in a drying chamber if it stays for a long time at a high temperature (this concerns powder deposited anywhere in the machinery). Ignition may already occur at 140°C; at 220°C, the time needed for spontaneous ignition is about 5 min. Hence, a dequate ventilation must be provided to maintain the concentration at low level.

32.7 Cooling the Powder

The dried product should be removed from the drier as quickly as possible after it is produced to minimize the effect of heat damage on the product. The product and air may be removed together from the drier and separated out side of the drier to reduce heat-effect.

Product cooling is done to prevent clumping, sticking and heat damage to the product. Prolonged heating causes staleness in nonfat dry milk, and causes fat to melt and move to the surface of whole milk powder. With more of the fat on the surface of the powder, the product will not keep as well in storage. Warm powder will hold the heat for some time in a bulk container, thus increasing the heat damage. The thermal conductivity, or k-value, is estimated at 0.03 Btu per hr sq ft °F per ft., which is considerably lower than most food particles and very similar to insulation materials.

Some cooling of the product will take place in the drier when using an air brush supplied with cool air to remove the dried product from the sides and bottom of the drier. The three principles of cooling powder outside the drier involve:

(1) Conduction cooling in which the product is cooled when moving through water jacketed screw conveyor.

(2) Convection cooling by using room air or refrigerated air to cool to 38°C or by moving conditioned air over the product or through the conveyor handling the product.

(3) Radiation cooling by placing a cold evaporator surface in view of the warm product. This method has not been exploited by the dry milk industry.

The outlet of a cyclone separator can be surrounded by a chamber through which cold-air is moved to cool the product. The material moves on to an entrainment separator for separation of solids and air. A vibrating conveyor for moving dry milk permits cooling as the product moves through the surrounding air. Oxygen can be removed more easily from warm powder than cold, when an inert gas, such as nitrogen is used for packing under vacuum. The amount of heat to be removed is given in Calorie by multiplying the specific heat by the weight, in kgs times the difference in temperature (°C). The density of drum dried milk is 0.3 to 0.5 gm per ml and for spray dried is 0.5 to 0.6 gm per ml. Some drying occurs in the cooling process. About one third to one half of the heat removed in the cooling process can be considered to be used for vaporization of water.
Last modified: Monday, 22 October 2012, 7:05 AM