There are at least two key facts to remember about mixing and blending, whether for solids in solids, liquids in solids, gases in liquids, or liquids in liquids. These are: 1) there is an optimum speed and time for any given system and 2) mixers need some head space or freeboard.
How do we know there is an optimum? In any process or unit operation, if one considers extremes of possible conditions or parameters, and at the extremes, there are unsatisfactory results, then there is some intermediate condition that is better. Most mixers rely on applying physical energy in some form to the components in order to move them around until the average composition is close to uniform. If no energy is applied, there is no movement and no mixing. If too much energy is applied, by running an agitator very fast, for instance, the contents are likely to just spin without much mixing. For example, a vortex or whirlpool may form in a liquid mixer.
It has also been observed empirically that mixing solids too long can cause a mixture to come apart or to segregate. Both speed and time, therefore, are amenable to optimization. Often they are combined in the dimensionless term, Nt, where N is rotational speed in reciprocal time units (seconds or minutes, usually) and t is mixing time in the same units. The term is equivalent to the number of turnovers or rotations applied and can be a useful scale-up parameter.
Accurate scale-up requires geometric similarity, which means using the same geometry and relative size of equipment. As we discuss later, there is a wide variety of available equipment for mixing, most of which is available in various sizes. Another useful parameter is power per unit volume (hp/gal is one way of expressing this term).
Head Space Requirements
Why do we need head space? The power or energy that is applied in mixing is used to move the contents, whether they are liquid or solid. There needs to be a place for the contents to go when they are agitated. It is simple physics, but this elementary fact is often overlooked in practice. Operators, always under pressure to increase throughput, see empty volume in a mixer as a wasted opportunity and inevitably try to use it by increasing batch sizes.
In addition to needing space in which to move, space or volume is needed to allow for expansion due to air entrainment in solids and foaming in liquids. Mixers need to be filled at least far enough to submerge the agitator, but normally should not be filled more than about 60% of theoretical volume. This is another illustration of the first point—at too low a volume, and at too high, mixing is inefficient, so there exists an optimum. The curves of many performance parameters against operating conditions are often relatively flat in the sense that a range of conditions gives good results. In the case of fill volume against mix time, for instance, fill volumes of 40–60% give similar results for many systems and equipment designs. Below or above this range, mixing may not occur or may take a long time.
Some Solids Mixers
I used to advocate a rule of thumb that said, in essence: “Tell me why a ribbon (or paddle—they perform similarly) mixer will not work.” These are machines that have a cylindrical body with straight sides and one or two horizontal shafts on which are arranged either two helical strips of metal or a series of paddles with slanted blades. As the shafts rotate, the ribbons or paddles move the contents in a characteristic pattern—toward one end on the outside of the chamber and toward the opposite end closer to the shaft.
Usually these mixers have a discharge port in the center of the bottom or at one end. The drive motor is outside and power is delivered by a belt. The shafts are supported by bearings and a seal at the drive end. These mixers are usually operated in batches, but can be made continuous. (More on that later.) Contents may be delivered pneumatically from bulk silos or intermediate bulk containers (IBC or big bags). Often, minor or micro ingredients are dumped from 1.5 cubic foot bags or drums. Small quantities may be scaled or made into a premix off-line.
It is worth noting that much of a mixing cycle—formulating, mixing, and dumping—may be occupied by the formulating or ingredient delivery step. During this time, the mixer is not mixing; it is just a receiver. One approach to increasing production is to do the formulating into another vessel or bin while the mixer is mixing the previous batch. Likewise, it is more efficient to dump into a second bin rather than to fill containers from the mixer. However, this approach requires additional head room to accommodate all the bins and so may be difficult to retrofit in an existing plant. Well-designed mixing plants often have towers to maximize mixer utilization.
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Ribbon or paddle mixers are relatively inexpensive and versatile. There are many suppliers. Design issues that need to be considered include ease of cleaning, ease of emptying (the bottoms are flat so contents may not drain completely), and ease of maintenance (the seals are subject to wear from abrasive ingredients). Dust collection is usually necessary where ingredients are dumped.
The Bella mixer from Dynamic Air, St. Paul, Minn. (www.dynamicair.com), has two counter-rotating shafts with paddles that are said to create a fluidized zone where mixing is intense.
The Littleford mixer from Littleford Day, Florence, Ky. (www.littleford.com), has both separately powered chopper blades and rotating plows in a cylindrical vessel. As is true of many other mixers, the vessel can be heated or cooled with a jacket and operated under a vacuum. This design has been effective in incorporating high quantities of fat in dry mixes, such as soup and sauce bases.
Amixon, Memphis, Tenn. (www.amixon.com), offers a range of agitated mixers in which one or two vertical shafts drive helical ribbon agitators. Access is through large side doors.
A novel solids mixer is the Rollo mixer from Continental Products Corp., Milwaukee, Wis. (www.continentalrollomixer.com). This machine is a large shallow cylinder that rotates on a horizontal shaft. It has lifting flights that scoop up and divide the contents, pouring them in a thin stream past a spray nozzle. It was originally developed to add molasses to animal feeds, but has found numerous other applications in which liquids are added to solids (free-fall processing). Rotation is slow, about 3.3 RPM, so wear on the mixed ingredients is negligible. Addition and removal of the contents is through chutes on the front hood; for removal, the falling curtain is intercepted as the drum rotates.
Other solids mixers include rotating V-shaped vessels, rotating double-cone vessels, and fluidized beds. These are especially useful when particles in a mix are fragile and must be treated gently.
In general, it is difficult to mix thoroughly particles with widely varying particle size or density. If possible, it is good practice to mill ingredients of a solids mix so they have similar particle size. If widely varying particles must be included in a mix, it may be more practical to fill them separately into a package or just assemble them on a conveyor belt supplying packaging. This is done for such products as trail mix or bridge mix—snacks with disparate pieces.
Continuous Mixing of Solids
Continuous mixing is a feeding challenge, not a mixing problem, normally. Various devices may be used to do the mixing. One example is a rotating series of Vs fed at one end and discharged at the other. Ribbon and paddle mixers can be fed continuously and discharged from the opposite end. In any case, it is the accuracy and consistency of the feeders that determines the quality of the mix.
Solids feeders depend on the physical properties of the powder to control flow. The critical properties are density, cohesiveness, and angle of repose. Density of powders can vary as air is entrained when they flow through lines or chutes. Feeders can be volumetric or gravimetric. If they are volumetric, then varying density will cause variation in flow weights.
Angle of repose refers to the angle with a horizontal plane that a pile of powder forms when poured from a given height. A steeper angle indicates a more cohesive powder. Cohesiveness can also be detected by pressing a sample between fingers. A cohesive powder forms a disk, while one that is less cohesive does not. Cohesive powders have a tendency to form arches at bin outlets and stop flowing. Powders at rest can be induced to flow by vibration, so often feed bins have vibrators attached.
Gravimetric feeders may use gain in weight or loss in weight. In either case, a receiver is mounted on load cells. There needs to be a flow control device—a screw, a star wheel, or a vibratory conveyor. The device is operated by a signal from the load cells. Care must be taken that solids do not flood or choke the feeder.
by J. Peter Clark,
Consultant to the Process Industries, Oak Park, Ill.