Dairy Engineering

Module 2. Bottle & cans washing and CIP cleaning equipment

Lesson 8
MECHANICAL SEPARATION: FUNDAMENTALS INVOLVED IN SEPARATION

8.1 Introduction

Separation is one of the important unit operations practiced in Dairy and Food processing. Some of the physical separation processes are Sieving, Filtration, Membrane separation, Gravity separation, Centrifugal Separation etc. They may also be classified broadly into categories of Liquid- Liquid, Liquid – Gas, Solid – Solid, Solid- Gas separations. Each operation would involve specific application for separation.

Some of the common operations in dairy industry are the separation of fat from milk, separation of extraneous matter from milk, separation of fine powder particles from exhaust air of spray drier, filtering moisture from various dairy products etc.

Filtration is the process of passing a fluid containing suspended particles through a porous medium. The medium traps the suspended solids producing a clarified filtrate. Filtration is employed when the valuable component of the mixture is the filtrate.

8.2 Gas-Solid Seperations

Gas-Solid separation involves the application of particle mechanics to the design and application of dust-collection systems. It includes wet collectors, or scrubbers, for particle collection. It could involve equipment for removing entrained liquid mist from gases.

8.2.1 Purpose of dust collection

Dust collection is concerned with the removal or collection of solid dispersed in gases for purposes of:

1. Air-pollution control, as in fly-ash removal from power-plant flue gases

2. Equipment-maintenance reduction, as in filtration of engine intake air.

3. Safety or health-hazard elimination, as in collection of siliceous and metallic dusts around grinding and drilling equipment and in some metallurgical operations and flour dusts from milling or bagging operations

4. Product-quality improvement, as in air cleaning in the production of pharmaceutical products and photographic film

5. Recovery of a valuable product, as in collection of dusts from dryers and smelters

6. Powdered-product collection, as in pneumatic conveying; the spray drying of milk, eggs, detergent and the manufacture of high purity zinc oxide and carbon black

Measurements of the concentrations and characteristics of dust dispersed in air or other gases may be necessary (1) to determine the need for control measures, (2) to establish compliance with legal requirements, (3) to obtain information for collector design, and (4) to determine collector performance.

The basic operations in dust collection by any device are (1) separation of the gas-borne particles from the gas stream by deposition on a collecting surface; (2) retention of the deposit on the surface; and (3) removal of the deposit from the surface for recovery or disposal. The separation step requires (1) application of a force that produces a differential motion of a particle relative to the gas and (2) a gas retention time sufficient for the particle to migrate to the collecting surface. The principal mechanisms of aerosol deposition that are applied in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition.

8.2.2 Cyclone separators

The most widely used type of dust collection equipment is the cyclone, in which dust-laden gas enters a cylindrical or conical chamber tangentially at one or more points and leaves through a central opening . The dust particles, by virtue of their inertia, will tend to move toward the outside separator wall, from which they are led into a receiver. A cyclone is essentially a settling chamber in which gravitational acceleration is replaced by centrifugal acceleration. At operating conditions commonly employed, the centrifugal separating force or acceleration may range from 5 times gravity in very large diameter, low-resistance cyclones, to 2500 times gravity in very small, high-resistance units. The immediate entrance to a cyclone is usually rectangular.

Fig. 8.1 Cyclone separator

8.2.3 Venturi scrubbers

The venturi scrubber is one of the most widely used types of particulate scrubbers. The designs have become generally standardized, and units are manufactured by a large number of companies. Venturi scrubbers may be used as either high- or low energy devices but are most commonly employed as high-energy units. The units originally studied and used were designed to the proportions of the classical venturis used for metering, but since it was discovered that these proportions have no special merits, simpler and more practical designs have been adopted. Most “venturi” contactors in current use are in fact not venturis but variable orifices of one form or another. Any of a wide range of devices can be used, including a simple pipe-line contactor. Although the venturi scrubber is not inherently more efficient at a given contacting power than other types of devices, its simplicity and flexibility favor its use. It is also useful as a gas absorber for relatively soluble gases, but because it is a concurrent contactor, it is not well suited to absorption of gases having low solubilities.

Current designs for venturi scrubbers generally use the vertical down flow of gas through the venturi contactor and incorporate three features:

(1)a “wet-approach” or “flooded-wall” entry section, to avoid dust buildup at a wet-dry junction; (2) an adjustable throat for the venture (or orifice), to provide for adjustment of the pressure drop; and (3) a “flooded elbow” located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractory lining to resist abrasion by dust particles.

Fig. 8.2 Venturi scrubber

8.3 Liquid-Solid Sepeations

These operations involve, the unit operations like Crystallization, Leaching, Gravity sedimentation, Filtration, Centrifugal separation etc. Of these Filtration and Centrifugal separation involve mechanical action in separation.

Sedimentation is the partial separation or concentration of suspended solid particles from a liquid by gravity settling. This field may be divided into the functional operations of thickening and clarification. The primary purpose of thickening is to increase the concentration of suspended solids in a feed stream, while that of clarification is to remove a relatively small quantity of suspended particles and produce a clear effluent. These two functions are similar and occur simultaneously and the terminology merely makes a distinction between the desired primary process results. Generally, thickener mechanisms are designed for the heavier-duty requirements imposed by a large quantity of relatively concentrated pulp, while clarifiers usually will include features that ensure essentially complete suspended-solids removal, such as greater depth, special provision for coagulation or flocculation of the feed suspension, and greater overflow-weir length.

8.3.1 Clarifiers

Continuous clarifiers are generally employed with dilute suspensions, principally industrial process streams and domestic municipal wastes, and their primary purpose is to produce a relatively clear overflow.

They are basically identical to thickeners in design and layout except that they employ a mechanism of lighter construction and a drive head with a lower torque capability. These differences are permitted in clarification applications because the thickened pulp produced is smaller in volume and appreciably lower in suspended solids concentration, owing in part to the large percentage of relatively fine (smaller than 10 mm) solids.

Fig. 8.3 Clarifiers

8.3.2 Rectangular clarifiers

Rectangular clarifiers are employed primarily in municipal water and waste treatment plants, as well as in certain industrial plants, also for waste streams.

8.3.3 Circular clarifiers

Circular units are available in the same three basic types as single-compartment thickeners: bridge, center-column, and peripheral-traction. Because of economic considerations, the bridge-supported type is limited generally to tanks less than 20 m in diameter.

8.3.4 Tilted-plate clarifiers

Lamella or tilted-plate separators have achieved increased use for clarification. They contain a multiplicity of plates inclined at 45 to 60° from the horizontal. Various feed methods are employed so that the influent passes into each inclined channel at about one-third of the vertical height from the bottom. This results in the solids having to settle only a short distance in each channel before sliding down the base to the collection zone beneath the plates. The clarified liquid passes in the opposite direction beneath the ceiling of each channel to the overflow connection.

Fig. 8.4 Tilted plate clarifiers