Separation Processes

Physical separations are applied to foods and ingredients to improve appearances, remove impurities, and improve performance. Here we will not consider those separations that involve a change of phase, such as distillation, evaporation, drying, and crystallization. Other physical separations include filtration, centrifugation, sedimentation, elutriation, magnetic separation, and membranes (a special case of filtration). When are they used? What sort of equipment is required?

Filtration
Filtration is used to remove solids from a liquid by passing a suspension through a porous cloth or screen. Depending on the pore size, solids larger than the pores are retained and build up a cake while the clear liquid and smaller solids pass through. One characteristic of classic filtration is that once a cake is formed, it becomes the filter medium and may be more restrictive than the clean cloth. Thus in some applications, the first volumes of filtrate may be recycled until it is as clear as desired.

An example of such an operation is lautering in brewing of beer. A suspension of ground malted barley in hot water, the mash, is circulated through a relatively coarse screen until a bed of barley husks builds up on the screen. The soluble sugars are dissolved in the hot water to make a clear wort, which, after cooling, is fermented. (There are several subtle variations in brewing, but the concept is the same.)

As the cake builds up in filtration, either the flow rate falls, at constant pressure, or the pressure must be increased to maintain the flow rate. Increasing pressure may cause the cake to compress, depending on the physical characteristics of the solids. Even relatively hard particles may compress somewhat as pressure forces them closer together. This mode of operation quickly becomes self-limiting, as increased resistance to flow, due to increased pressure, requires ever-increasing pressure, which increases resistance. Eventually, the operation must be stopped, the cake removed, and, possibly, the filter cloth or screen cleaned.

In the early stages of batch filtration, fine particles may enter the pores of the cloth or screen, partially plugging them and adding to flow resistance. For that reason, sometimes a pre-coat is applied to the cloth or screen, consisting of inert solids that are large enough to form a cake without plugging the cloth. Rice hulls, cellulose fibers, and diatomaceous earth are common pre-coat materials used in food processing.

When the solids are the desired product, a pre-coat may not be appropriate. An example of such a case is chocolate processing, in which finely ground, roasted cocoa nibs are filtered from liquid cocoa butter to produce a cake of cocoa powder and clear cocoa butter, the more valuable component. Cocoa butter is mixed with sugar, some cocoa powder, milk, and emulsifiers to make chocolate. Excess cocoa powder is an additional product, and may be further processed to make drinks, ingredients for baked goods, and flavors for ice cream.

In addition to relatively simple batch pressure filtration, there are numerous variations to approach continuous operation, by periodicallyor continuously removing the filter cake. Vacuum drum filtration occurs on a large, rotating, porous drum covered with a cloth and capable of drawing a vacuum from the inside. A portion of the drum is submerged in a pan containing the suspension of solids. As the drum rotates, a cake is formed and drained. There may be a washing step to displace entrained liquid. Just before re-entering the feed pan, a doctor knife scrapes off the cake. By applying suspension through a nozzle, a vacuum drum filter may create a pre-coat.

An example of such an operation is the use of activated carbon as a pre-coat on a vacuum drum filter to salvage dissolved sugar from recycled candy in a confectionery plant. The carbon removes colors and flavors, yielding a clear syrup that can be reused to make candy. This process not only saves money, it also reduces potential waste.

--- PAGE BREAK ---

Centrifugation
Sedimentation is the separation of materials under the force of normal gravity. The rate at which a solid particle will fall in a liquid is a function of the difference in densities between particle and liquid, as well as the apparent diameter of the particle and the viscosity of the liquid. In fact, this phenomenon can be used to determine the viscosity of certain liquids, knowing all the other parameters. The technique is known as falling ball viscometry. Small particles with densities close to that of water may take a long time to settle, making simple sedimentation often impractical for clarifying liquids, such as juices. On the other hand, while slow, sedimentation may be a nuisance in some cases, as when desired suspensions settle on standing. One solution to undesired sedimentation is to increase the viscosity of the liquid phase, perhaps with gums. Another approach is to finely grind the particles.

Grape juice and wines are clarified by holding for long periods of time to allow tartaric acid and tartrate salts first to crystallize and then to settle into lees, which can be harvested as a valuable by-product.

Centrifugation relies on high speed rotation of a chamber to apply many times the normal force of gravity to separate materials of different densities. Sometimes the solids are desired, while in other cases, the objective is to clarify the liquid. Different objectives lead to different equipment designs.

Centrifuges may be horizontal or vertical, batch or continuous, and usually have provisions to adjust where the phases are removed.

A simple batch centrifuge consists of a vertical solid bowl that can be fed a suspension near the center rotating shaft. Solids collect at the wall and clear liquid collects near the center and may be periodically removed while suspension is added until the bowl is partially full of solids. Water may be added to wash the solids, and then the solids are removed by a plow while the bottom of the bowl is opened.

An example of this operation is recovery of sugar crystals from a suspension created by evaporation of sugar cane juice or sugar beet extract. The liquid, or mother liquid, is molasses and is typically reprocessed until no more sugar can be recovered. Sugar crystals typically still have some impurities after one or two separations and are refined by re-dissolving and re-crystallizing in water until they are over 99% pure sucrose. The sequence of loading, washing, and discharging is often automated in a sugar mill or refinery.

Some centrifuges can produce three phases as in rendering, where meat scraps and bones are cooked in steam to release fats. A suspension of solid protein particles, liquid fat (tallow from cattle or lard from hogs), and an aqueous phase, known as stick water, is fed to a horizontal centrifuge with an internal screw auger that transports solids from the wall to a discharge port. The aqueous and fat phases are separately discharged continuously. The solids are high in protein and may become animal feed ingredients; the fat may be edible or inedible, depending on how the feed material has been treated; and the stick water is usually concentrated as a source of meat flavors or a feed ingredient. If it were not for the profits from rendering, meat packing would rarely be profitable.

A variation on this pocess of edible rendering is the source of finely textured lean beef, used as an ingredient in some ground meats. The protein from inexpensive fatty beef trim is concentrated by just barely melting the trim and then centrifuging to separate the protein, which then is quickly frozen. Edible tallow is an additional product.

--- PAGE BREAK ---

Separating Particles in Air
Dust collection is an important utility in many food plants. Typically the objective is to reduce potentially hazardous dust that may pose an explosion or toxicity threat. Many food dusts are combustible or flammable in the right conditions of concentration and relative humidity, coupled with an ignition source, such as an electric spark. Grain and flour dusts are particularly notorious, but sugar dust also poses a threat. Dusts need to be suspended to explode, but after an initial explosion, accumulated dust may be re-suspended and cause a larger and more damaging explosion.

Dust collection systems provide hoods and other openings to a vacuum conveying duct that leads to a central cyclone and filter vessel. The face velocity at the collection points must be carefully tuned to suspend dust without removing too much of the desired product. Typical collection points are bag dump stations, mixers, and other places where dusty ingredients or mixes are exposed. Velocity in the ducts must keep the particles suspended. Many dust collection systems get modified over time or were improperly designed, so horizontal collection ducts become filled with settled dust. This can create a haven for insects and pose a potential explosion hazard if disturbed.

A good dust collection system has provisions for opening and inspecting ducts.

At the collection vessel, a cyclone helps remove some dust by whirling the air within a conical vessel. The solids drop to a drum for disposal. Discharged air passes through cloth bags or polymer cartridges before release to the atmosphere.

J. Peter Clark, Contributing Editor,
Consultant to the Process
Industries, Oak Park, Ill.
[email protected]