Characterization of the performance of bin blenders: part 1 of 3: methodology: in this series of articles, bin blender performance is comprehensively reviewed using both free-flowing and cohesive mixtures. In part 1, an introduction to tools and techniques is presented, followed by an examination of parameter effects, mixing mechanisms, and the effects of cohesion on mixing
Blending powder and granular constituents plays a vital role in the production of a wide array of consumer and industrial products, including ceramics, plastics, foodstuffs, and pharmaceuticals. Among the available equipment for powder mixing, tumbling blenders remain the most prevalent. A number of different geometries are available from blender manufacturers, including V-blenders, cube blenders, and double cones. However, a more recent addition to tumbling blender geometries is the bin blender, which is also known as an intermediate bulk container (see Figure 1). The bin blender was originally designed so that after blending is completed the container can be removed from the drive and transported to the next process of operation without discharging its contents into a secondary vessel (i.e., hopper, barrel, etc.). This added functionality eliminates the need for additional transport containers and avoids tying up the production line. Furthermore, this design minimizes operator contact with the blender contents, which can be hazardous. This series of three articles provides an overview of recent computational and experimental findings regarding the performance of bin blenders over a range of processing conditions and mixture types.
Characterization of mixing processes
Mixing in tumbling blenders. The simplest function of a tumbling blender is to blend all the constituents of a given mixture in a single processing step. In this function, each ingredient is loaded separately into the blender, and the blender rotates until a homogenous mixture has been formed. The tumbling blender also may be used to blend lubricants into an already homogenous powder mixture. In addition, tumbling blenders can be used as preblenders for mixing a low-dose active ingredient (often cohesive) with a portion of the excipients. Once this preblending step is completed, the mixture then is transferred to a larger blender (tumbling, convective, pneumatic, etc.) and mixed with the rest of the excipients before further processing.
Many experimental investigations regarding the performance of tumbling blenders have appeared in the literature over the past few decades. Some studies have used broad comparisons of the utility of different blender types (2 or more at a time) for one of two particular mixtures (1-3). Other studies have investigated the mixing efficiency of one or two blenders for multiple mixtures to compare efficiency (4-6). Only a few studies have used a single blender with a single mixture to determine the effects of various operational parameters on blender efficiency (7-10).
Bin blenders have only recently been specifically examined (11-13). However, these studies found bin blenders to be similar in geometry and functionality to double cone blenders, which have been more extensively covered (8, 14-17). Essentially, a bin blender is a "single cone blender"--a double cone blender cut in hall. In the investigations of double cones, radial mixing (i.e., perpendicular to the axis of rotation) has been found to be more than an order of magnitude faster than axial mixing (parallel to the axis of rotation). Furthermore, inserting baffles to increase axial displacement has been shown to markedly increase mixing rates. These generic characterizations of double cone blender performance are similar to the mixing performance in bin blenders for some materials and processing conditions. However, for other mixtures, certain baffle configurations, and processing conditions, differences in blending performance occur, which will be discussed in detail throughout this article.
Sampling tools and methods
The opacity of granular materials often requires the extraction of spot samples for compositional analysis to determine mixture quality. Currently, the characterization of granular mixtures is limited by the errors and biases associated with most available means of sample extraction. The most commonly used devices for sample retrieval are thief probes. End-sampling and side-sampling thieves have been shown to produce erroneous information regarding spot sample compositions (18, 19). A major problem with most thieves is that the retrieved sample is not representative of the true concentration at the location from which the sample was supposed to be obtained. These sampling errors are caused by contamination with material from other locations in the mixture during probe insertion. Also, nonuniform flow of different components into the sampling cavity can skew the sample concentrations (which is common when different size particles are present). Recent work has shown that two samplers--the groove sampler and the core sampler--which are used exclusively in this article, are more effective, accurate, and reliable than typical side-sampling or end-sampling thieves (19, 20).
The groove thief consists of a hollow sleeve (1 in. in diameter) surrounding a solid inner steel rod with a groove bored along most of the length of the pipe (19). The inner pipe has a sampling cavity that is ~1/2 in. deep and wide along the middle 80% of the rod. Rotating the inner pipe relative to the outer pipe opens and doses the sampler. The sampler is inserted into the powder bed while open; rotating the inner tube traps material within the sampler (see Figure 2a-c). After being removed from the powder bin, the sampler is then placed horizontally on a stand while open, and the entire device is rotated to discharge the collected material into a series of small trays (see Figure 2d). Sample size can vary depending on the size of the sampler or the width of the containers into which the material is discharged.
[FIGURE 2 OMITTED]
The other sampling technique uses a core sampler (a hollow tube filed to a thin edge at one end) to gather samples. The tube is thrust into the mixture and retrieved, leaving a core of material in the sampler that is held in place by static friction forces, and is then extruded in a last-in--first-out manner (see Figure 3). The use and accuracy of this sampler has been described extensively (20). This sampler has proven to give more-accurate representations than typical thief probes of mixture distributions while simultaneously causing less disturbance of the powder bed.
[FIGURE 3 OMITTED]
Avoiding contamination during sample collection is vital, but determining the location and number of samples to extract from the mixture is equally important. Often, samples are taken from throughout the bed to ensure complete coverage of the entire mixture. Although this approach guarantees thoroughness, it can lead to wasted time, effort, and material if more efficient means are available.
Mixing in tumbling blenders is often limited by the axial transfer of material or by segregation of the components (usually caused by variations in particle characteristics such as size or shape). Previous work has shown that segregation of mixtures in some tumbling blenders creates axial gradients in concentration (16, 21). Similar results occur in a bin blender when a binary-distributed mixture of glass beads is run at constant rotation rate. Figure 4 shows the three types of segregation patterns that form in a binary mixture of 1.6mm and 600[micro] glass beads when run at different rotation rates. These segregation patterns correspond exactly to those seen in double cone blenders over a wide range of rotation rates and particle sizes. The mechanisms and effects of particle size and particle size ratio have been discussed in detail (16).
[FIGURE 4 OMITTED]
These data imply that axial sampling of the blender is vital whereas radial sampling (sampling at multiple locations on the same line perpendicular to the axis of rotation) may be superfluous. This concept has been tested in a 56-L bin blender. Figure 5a shows a typical total sampling scheme for a circular opening using 14 core sampler locations. In Figure 5b, the variance measured using only the axial samples (i.e., cores 1, 8, 9, 12, 13) is compared to the results obtained from using all the probes. Although the number of samples has been reduced by almost a factor of 3, the resulting variance versus revolutions data show very good agreement, indicating that limited axial sampling gives information equivalent to that obtained from total sampling (19).