Lessons in Mixing

Mixing, especially of solids, is a common and important unit operation in food processing but is often poorly understood. The following discussion draws from various practical cases and is based in part on the Food Engineering Div. Lecture that I was privileged to deliver at the IFT Annual Meeting & Food Expo^SM last month. I reviewed some of the key issues in solids mixing: material handling, proper mix time, mixer volume, scheduling and surge, segregation, and feeding, especially in the case of continuous mixing.

• Material Handling. This refers to the delivery, scaling, and conveying of various components of a mixture, typically solids such as flour, sugar, salt, or other ingredients. Most formulas have major, minor, and micro ingredients, distinguished by their relative amounts in the formula. The dividing points are matters of judgment, but one approach is to say that amounts >10% are major, 1–10% minor, and <1% micro ingredients. Micro ingredients, while occurring in relatively small amounts, are often critical to the functionality of the final mix, as they may be vital nutrients, leavening agents, colors, or flavors. Thus, their proper dispersion in the mix may be especially crucial.

Each category of ingredient may be delivered in a different way. For example, major ingredients are often delivered in bulk, unloaded pneumatically into bins, and delivered to a mixer by pressure or vacuum pneumatic lines. Pneumatic transfer refers to conveying solids in pipes or tubes by suspending the solid particles in moving air. Depending on the concentration of solids in air, the transfer may be considered dense phase or dilute phase.

Pneumatic conveying can affect the solids in various ways, not always beneficial. For example, fragile particles may be broken by impact with elbows in the conveying line. Crystalline sugar is especially vulnerable and is therefore often conveyed in dense phase to reduce creation of fines.

The act of compressing air to convey solids raises the temperature of the air and increases its relative humidity, both of which may change the properties of the solids. Also, it may be inefficient to draw air from within the plant, because that air may have been heated or cooled; therefore, good practice is to extend the inlet of the compressors or blowers outside. Since the machines are also noisy, it is common to put them in a separate room with an outside wall.

Pressure conveying is convenient for delivering solids from one source, such as a storage bin, to multiple use points. Vacuum conveying is useful for delivering solids from multiple sources to one use point; it is often used for unloading delivery trucks or rail cars.

Minor ingredients are often delivered in bulk bags, smaller bags, or drums. Bulk bags, large cloth or polymer containers, may hold hundreds of pounds, while smaller bags rarely exceed 100 lb, depending on the bulk density. In any case, these ingredients must be removed from their container, weighed, and delivered to the mixer. Typically, this is done at bag dump stations that have dust-collection hoods above a safety grid on which the bags are placed and slit; the solids are then conveyed to a bin or the mixer.

• Scaling. Sometimes formulas are deliberately adjusted to use an integral number of bags of ingredients, but rarely can this be arranged for every component. Thus, there is always a need for weighing or scaling. This might be done off-line by hand (especially for micro ingredients), or it might be done by placing the mixer or a bin on load cells. Alternatively, ingredients might be loaded into bins and amounts weighed by loss in weight of the bin as the bin is emptied.

If each ingredient is separately weighed, then ingredients can be loaded simultaneously into the mixer, shortening the time between batches. However, this requires more bins and scales. As discussed below, there is always a trade-off between capital cost, labor, time, and automation.

Ergonomic considerations now dictate that people not be required to lift vertically more than 40 lb, so bag dump stations are usually provided with scissors lifts to elevate bags to the same height as the bag dump. In addition to controlling the dust from dumping, provision must be made to remove the empty bags. Sometimes, bag dumping is conducted in a separate area to reduce the potential for contamination in the food processing area.

• Mix Time. Mixing is a case where more is not necessarily better. There is usually an optimal mix time, which must be determined experimentally. The experiment is tedious, because mixing is determined by measuring the standard deviation of some critical component. This requires taking multiple samples, at least ten, from various parts of the mixer at a succession of times. Often, mixing times are determined by using an easy-to-analyze component, such as salt, but care must be taken that the results apply to the material of most interest, since it may have different particle size and density than salt does.

If mix time is determined on small-scale equipment, scale-up parameters can be established by using similar geometric ratios and keeping the Froude number constant. The Froude number is a dimensionless group equal to n²d/g, where n is rotational speed, d is a characteristic dimension of the mixer such as diameter, and g is the acceleration due to gravity, all in consistent units.

The implication of this approach is that as a mixer of the same geometric ratio, such as length to diameter, gets larger (i.e., the diameter gets larger), the required rotational speed is reduced to keep the Froude number constant. The resulting mix time in a larger mixer might actually increase, because the intention is to keep the number of turnovers N_t, where t is the mix time, constant.

• Mixer Volume. Most solids mixers have working volumes equal to 50% of their total volume. This means, for instance, that a ribbon or paddle blender should only be filled to just above its shaft. Other designs, such as tumblers and V-blenders, likewise must be only partially filled. This can be a source of friction between operators who want to maximize batch sizes and developers who understand the limitations of the equipment. If a mixer is overfilled, it is difficult for the solids to be moved by the agitator and mixing will be poor at best.

Net production is actually improved by using equipment properly, because mix times will be shorter and quality higher, resulting in less rework.

• Scheduling and Surge. The simplest mixing process consists of loading directly into a mixer, mixing for the correct time, then dumping directly into packages or processing further. This requires some elevation of the mixer, to allow room for packaging or conveying under the equipment. Often, there is a work platform on which bagged ingredients are placed and an operator manually loads the mixer. The mixer is idle during loading and unloading.

A modest improvement in cycle time is achieved by delivering some ingredients in bulk to a receiver and scale, but this requires more head room and additional equipment.

The next level of complexity adds a receiving bin, into which the components of a formula are loaded, manually through a bag dump station or pneumatically, while the previous batch is being mixed. The mixer is idle during unloading but is loaded quickly from the receiving bin once it is empty. The assumption is that successive batches of the same formula or of compatible formulas are being made. If cleaning is required between batches, then that time is added to the cycle.

An obvious further improvement is to add a holding bin after the mixer. Now the mixer has minimal idle time, so production throughput, all other things being equal, is maximized. However, there is a price in both space and capital cost. Because solids transfers rely on gravity, this configuration is vertical and may not fit in an existing building. A common arrangement is to have bag dumps and controls at ground level, but a work platform is still needed for cleaning and maintenance.

In at least one instance, where packaging of a dry-mix consumer product was proposed to be increased, the addition of receiving bins was suggested to enable an existing mixer to keep up. It already had a holding bin for mixed product.

• Segregation. Mixing is usually just one step in a process. The mixed product may be further processed, as in baking for instance, or it may be a final product, such as a spice blend, cake mix, or food flavor. In any case, it must be packaged or conveyed elsewhere. This creates the opportunity for segregation, undoing the mixing that just occurred.

Segregation occurs because of differences in particle sizes and densities among the mixed components. Just the act of dropping a solid mixture through a chute can cause separation. Reducing velocity, baffling, and reducing drop distances can reduce, but not eliminate, segregation.

The surest solution is to make all particles as close to each other in size as possible. This might require milling or grinding of some ingredients, introducing another unit operation. Another approach is to agglomerate small micro ingredients onto larger major or minor ingredients by adding small amounts of a liquid, such as water, oil, or a volatile solvent. This requires that the added liquid be acceptable in the end use or that it be removed by drying. It also requires uniform spraying of the liquid and good mixing.

• Feeding. Finally, if a proposed mixture must have widely disparate particles, such as croutons in a seasoning mix for stuffing, or the confection called bridge mix, it might be most practical to avoid conventional mixing altogether and just assemble the product in the package. This then becomes a feeding challenge instead of a mixing operation. Feeding is also the critical factor in continuous mixing.

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