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河南理工大學萬方科技學院本科畢業(yè)論文
附錄：
外文資料與中文翻譯
外文資料：
Comparing mixing performance of uniaxial and biaxial bin blenders
Amit Mehrotra and Fernando J. Muzzio
Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway, NJ, 08855, United States
Received 17 February 2009;
revised 30 May 2009;
accepted 14 June 2009.
Available online 27 June 2009.
Abstract
The dynamics involved in powder mixing remains a topic of interest for many researchers; however the theory still remains underdeveloped. Most of the mixers are still designed and scaled up on empirical basis. In many industries, including pharmaceutical, the majority of blending is performed using “tumbling mixers”. Tumbling mixers are hollow containers which are partially loaded with materials and rotated for some number of revolutions. Some common examples include horizontal drum mixers, v- blenders, double cone blenders and bin blenders. In all these mixers while homogenization in the direction of rotation is fast, mediated by a convective mixing process, mixing in the horizontal (axial) direction, driven by a dispersive process, is often much slower. In this paper, we experimentally investigate a new tumbling mixer that rotates with respect to two axes: a horizontal axis (tumbling motion), and a central symmetry axis (spinning motion). A detailed study is conducted on mixing performance of powders and the effect of critical fundamental parameters including blender geometry, speed, fill level, presence of baffles, loading pattern, and axis of rotation. In this work Acetaminophen is used as the active pharmaceutical ingredient and the formulation contains commonly used excipients such as Avicel and Lactose. The mixing efficiency is characterized by extracting samples after pre-determined number of revolutions, and analyzing them using Near Infrared Spectroscopy to determine compositional distribution. Results show the importance of process variables including the axis of rotation on homogeneity of powder blends.
Graphical abstract
The dynamics involved in powder mixing remains a topic of interest for many researchers; however the theory still remains underdeveloped. Most of the mixers are still designed and scaled up on empirical basis. In many industries, including pharmaceutical, the majority of blending is performed using “tumbling mixers”. In all these mixers while homogenization in the direction of rotation is fast, mediated by a convective mixing process, mixing in the horizontal (axial) direction, driven by a dispersive process, is often much slower. In this paper, we experimentally investigate a new tumbling mixer that rotates with respect to two axes: a horizontal axis (tumbling motion), and a central symmetry axis (spinning motion).
Keywords:Powder mixing ; Cohesion; Blender ; Mixer; Relative standard deviation; NIR; Acetaminophen
Article Outline
1.
Introduction
2.
Materials and methods
2.1. Near infrared spectroscopy
2.2. Bin blenders used in this study: uni-axial blender (Blender 1), bi-axial blender (Blender 2)
2.3. Experimental method
3.
Results
4.
Conclusion
References
1. Introduction
Particle blending is a required step in a variety of applications spanning the ceramic, food, glass, metallurgical, polymers, and pharmaceuticals industries. Despite the long history of dry solids mixing (or perhaps because of it), comparatively little is known of the mechanisms involved [1], [2] and [3]. A common type of batch industrial mixer is the tumbling blender, where grains flow by a combination of gravity and vessel rotation. Although the tumbling blender is a very common device, mixing and segregation mechanisms in these devices are not fully understood and the design of blending equipment is largely based on empirical methods. Tumblers are the most common batch mixers in industry, and also find use in myriad of application as dryers, kilns, coaters, mills and granulators [4], [5], [6], [7] and [8]. While free-flowing materials in rotating drums have been extensively studied [9] and [10], cohesive granular flows in these systems are still not completely understood. Little is known about the effect of fundamental parameters such as blender geometry, speed, fill level, presence of baffles, loading pattern and axis of rotation on mixing performance of cohesive powders or the scaling requirements of the devices.
However, conventional tumblers, rotating around a horizontal axis, all share an important characteristic: while homogenization in the direction of rotation is fast, mediated by a convective mixing process, mixing in the horizontal (axial) direction, driven by a dispersive process, is often much slower.
In this paper, we experimentally investigate a new tumbling mixer that rotates with respect to two axes: a horizontal axis (tumbling motion), and a central symmetry axis (spinning motion). We examine the effects of fill level, mixing time, loading pattern and axis of rotation on the mixing performance of a free-flowing matrix of Fast Flo lactose and Avicel 102, containing a moderately cohesive API, micronized Acetaminophen. We use extensive sampling to characterize mixing by tracking the evolution of Acetaminophen homogeneity using a Near Infrared spectroscopy detection method. After materials and methods are described in Section 2, results are presented in Section 3, followed by conclusions and recommendations, which are presented in Section 4.
2. Materials and methods
The materials used in the study are listed in Table 1, along with their size and morphology. Acetaminophen is blended with commonly used excipients and is used as a tracer to evaluate the degree of homogeneity achieved as a function of number of revolutions. Acetaminophen is one of the drugs most widely used in mixing studies, and Avicel and Lactose are commonly used pharmaceutical excipients. In the interest of brevity their SEM images are not included in this paper, but can be found in “Handbook of Pharmaceutical excipients”.
2.1. Near infrared spectroscopy
Acetaminophen homogeneity was quantified using near infrared spectroscopy. A calibration curve was constructed for a powder mixture containing (in average) 35% avicel PH 102, 62% lactose and 3% acetaminophen. Near infrared (NIR) spectroscopy can be a useful tool to characterize acetaminophen. Samples are prepared by keeping the ratio of Avicel to lactose randomized in order to minimize effects of imperfect blending of excipients during the actual experiments on the accuracy of the results. The Rapid Content Analyzer instrument manufactured by FOSS NIR Systems (Silver Spring, MD) and Vision software (version 2.1) is used for the analysis. The samples are prepared by weighing 1 g of mixture into separate optical scintillation vials; (Kimble Glass Inc. Vineland, NJ) using a balance with an accuracy of ± 0.01 mg. Near-IR spectra are collected by scanning in the range 1116–2482 nm in the reflectance mode. Partial least square (PLS) regression is used in calibration model development using the second derivative mathematical pretreatment to minimize the particle size effects. As shown in Fig. 1, excellent agreement is achieved between the calibrated and predicted values.
Fig. 1.
Fig. 1. Near Infrared (NIR) spectroscopy validation curve. The equation used to predict acetaminophen concentration is validated by testing samples with known amounts of acetaminophen concentration. The y axis represents the concentration calculated from the equation and the x axis represents the actual concentration. Thus a perfectly straight line at 45° would represent the best calibration model. Each point on the graph represents a single sample. The concentration of acetaminophen examined here ranges from 0 to 8%.
2.2. Bin blenders used in this study: uni-axial blender (Blender 1), bi-axial blender (Blender 2)
Due to its widespread use, a cylindrical blender 1 with a capacity of 30 L is chosen as a reference blender in the study. As shown in Fig. 2, this blender has a circular cross section and tapers at the bottom. It can be used with or without baffles, which are mounted on a removable lid. In this study all the experiments are conducted without the use of baffles. Mixing performance in this device is used to provide a base-line for evaluating the mixing performance of a newly developed blender 2 with a capacity of 40 L, which is also cylindrical, in order to determine the effect of dual axis of rotation on mixing performance. The blender shown in Fig. 2(b) has two axis of rotation. The spinning rate of precession relative to the central axis of symmetry is geared to be half of that of the rate of rotation around the horizontal axis.
Fig. 2.
Fig. 2. Pictorial representation of (a) bin blender 1 and (b) bin blender 2 showing the corresponding axis of rotation.
2.3. Experimental method
Two types of initial powder loading used in the experiments: top–bottom loading and side–loading, which are schematically represented in Fig. 3. To avoid agglomeration, the API, acetaminophen, was delumped prior to loading it into the blender by passing it through a 35 mesh screen. In order to characterize mixing performance, a groove sampler was used to extract samples from the blenders at 7.5, 15, 30, 60, 120 revolutions. The thief was carefully inserted in the bin, and a core was extracted at each point of insertion (each “stab”) minimizing perturbation to the powder bed remaining in the blender. Approximately 7 samples are taken from each thief stab, and a total of five stabs are used at each sampling time, as shown in Fig. 4 so a total of 35 samples are taken at each sampling point.
Fig. 3.
Fig. 3. Schematic of the loading pattern used in the study. In top–bottom loading, Avicel is loaded first into the blender followed by Lactose on top of it and finally Acetaminophen is uniformly sieved over. In side–side loading avicel is placed at the bottom and then Acetaminophen is only sieved only in half part of the blender and is sandwiched between lactose and Avicel.
Fig. 4.
Fig. 4. (a) Thief sampler (b) top view of the sampling position scheme.
The experimental plan used in this study is as follows:
? Fill level: blender 1–60%
? Fill level: blender 2–60%, 70%, 80%
? Loading pattern: blender 1 — side–side loading, top–bottom loading
? Loading pattern: blender 2 — side–side loading, top–bottom loading
? Speed: blender 1–15 rpm, 20 rpm, 25 rpm
? Speed: blender 2 — rotational/spinning:15/7.5 rpm, 20/10 rpm, 30/15 rpm
? Sampling time: blender 1, blender 2–7.5, 15, 30, 60, 120 revolutions
3. Results
The homogeneity index used is the RSD, where C is the concentration of each individual sample, C_? is the average concentration of all samples and n is the total number of samples obtained at a given sampling time.
We examine the effect of fill level on mixing performance. Previously there have been studies on the effect of fill level in the Bohle bin blender, Gallay bin blender and V- blender and double cone blender [11], [12] and [13]. All the aforementioned blenders have only one axis of rotation, therefore the objective of this study is to examine how dual axis impact mixing performances at high fill levels. To avoid repetition, studies for fill level are not conducted for bin blender 1. Results available from a previous study using MgSt as a tracer showed that mixing in a uni-axial blender slowed down quite dramatically as the fill level exceeded 70%. Moreover, results for acetaminophen can be assumed to be similar to those obtained in previous work by Muzzio et al. [11] and [13], for a single axis rectangular bin blender [11], which have shown that even after few hundred revolutions homogeneity achieved with a 80% fill level is very poor as compared to 60% fill level.
To examine the effect of fill level in a dual axis blender, experiments were performed in blender 2 with the top-bottom loading pattern for a rotational speed of 15 rpm and with spinning speed of 7.5 rpm. The fill levels examined are 60%, 70% and 80% respectively and samples are taken after 7.5, 15, 30, 60, 120 revolutions. Typical results are shown in Fig. 5, which shows the RSD vs. number of blender revolutions. As expected for non-agglomerating materials, the curves show a rapidly decaying region. The slope of the curves in this region, in semi-logarithmic coordinates, is used to define the mixing rate. The curves then level off to a plateau that indicates the maximum degree of homogeneity that is achievable in the blender for a give material.
Fig. 5.
Fig. 5. Mixing curves for different fill levels in blender 2. The RSD of acetaminophen is plotted as a function of number of revolutions. The loading pattern in top-bottom and the blender rotational speed is 15 rpm with spinning speed of 7.5 rpm.
Similar to previous studies with other tumbling blenders we observe that blending performance is adversely affected by increasing fill levels. As shown in Fig. 5, the curve for 80% fill performs more poorly than those for 60% and 70% fill; as fill level increases, RSD curves decay more slowly, signifying a slower mixing process. However, the effect is not as pronounced as in other bin blenders and after about only 100 revolutions, the same plateau (the same asymptotic blend homogeneity) is achieved for all three fill levels.
Next, the effect of rotational speed is investigated in the blender 1 with one axis of rotation and is compared to the blender 2 with dual rotation axis. Experiments were conducted for both blenders with top-bottom and side-side loading. Experiments were performed at 60% fill level and the rotation speeds considered for blender 1 are 15 rpm, 20 rpm and 25 rpm respectively. As shown in Fig. 6 and Fig. 7, when plotted as a function of blender revolutions, there is not much of an effect of rotation speed on the homogeneity index (RSD) of acetaminophen at 60% fill level. It is observed that mixing performance at 20 rpm and 25 rpm is slightly better than at 15 rpm, however the differences in the performance of the blender under different speeds are probably too small to be significant. RSD curves decay with the same slope, indicating similar mixing rates. In the study reported here, the fill level is only 60%, and all the rotational speeds are enough to achieve homogenization. The aforementioned studies were conducted at 85% fill level. For such a high fill level, at low speeds, a stagnant core is known to occur at the center of many blenders, requiring higher shear stress per unit volume to achieve homogenization. Moreover, the flow properties of MgSt are known to be strongly different than those of most materials, and are known to have a deep impact on the flow properties of the mixture as a whole. Furthermore, MgSt is famously known to be a shear sensitive material. Thus an expectation that lubricated and unlubricated blends would show similar behavior with respect to shear is probably unwarranted.
Fig. 6.
Fig. 6. Mixing curves for top-bottom loading experiments with 60% fill level. RSD is plotted as a function of number of revolutions. Dotted lines correspond to experiments in the blender 1, while solid lines represent data points from the blender 2.
Fig. 7.
Fig. 7. curves for side–side loading experiments with 60% fill level. RSD is plotted as a function of number of revolutions. Dotted lines correspond to experiments in the while solid lines represent data points from the 2.
Subsequently, experiments were performed using the blender 2 at three rotation speeds: 15 rpm, 20 rpm and 30 rpm, and as explained before, the corresponding spinning speeds were 7.5 rpm, 10 rpm and 15 rpm. Fill level considered for both side-side and top-bottom loading was 60%.
Again, it was observed that varying rotation and spinning speeds did not make much difference in mixing rate. As shown in Fig. 6 and Fig. 7, mixing curves for blender 2 vary only slightly with rotation speed. For the top-bottom loading pattern it appears that mixing improves slightly when rotation speed is increased (the plateau is slightly lower for higher rotation speeds, indicating an improvement in the levels of asymptotic homogeneity), but no significant changes with speed are observed in side-side loading pattern.
The blending performance of both blenders is compared at different rotation speeds for both side-side and top-bottom loading patterns. To make a fair comparison, the fill level was kept as 60% for both blenders, a condition for which both blenders achieve effective mixing at long enough times. Due to geometric similarity of the two blenders, this comparison help evaluate the effect of spin (rotation with respect to the central symmetry axis) on mixing performance. As shown in Fig. 6, the mixing curves for the blender 2 lie below those for the blender 1 for each rotation rate, indicating faster mixing. Note that the final RSD asymptote reached for both blenders is also different, with the blender 2 showing a lower asymptote (better final mixed state, presumably due to a lesser effect of the slow mixing mode in the horizontal direction) than blender 1.
Similar results were obtained for the side-side loading pattern, as displayed in Fig. 7. The RSD curves for the blender 1 for all the three rotation rates lie above the blender 2. It is therefore confirmed that spinning a blender in direction perpendicular to the rotation axis helps in enhancing mixture homogeneity; however, for the materials examined here, the rotation rate does not have much effect on mixing performance.
Finally, a comparison is made between the two loading patterns for both blenders. Again, to achieve a fair comparison, all experiments are performed at 15 rpm and 60% fill level. As evident in Fig. 8, in both blenders top–bottom loading gives a more rapid decay of the RSD, indicating faster homogenization as compared to side–side loading pattern. However, for both loading modes, blender 2 achieves faster homogenization.
Fig. 8.
Fig. 8. Comparison between the mixing curves of the blender 2 and the blender 1 for top–bottom and side–side loading pattern. Dotted lines correspond to experiments in the blender 1, while solid lines represent data points from the blender 2. Experiments are performed at 15 rpm with 60% fill level.
As reported in previous studies, all the RSD curves in this paper exhibit a common trend with respect to time, characterized by an initial period of rapid homogenization due to convective mixing, followed by a period of much slower homogenization typically controlled by dispersion or shear. This trend is shown schematically in Fig. 9. The first regime is a fast exponential decay and the second one is a slow exponential asymptote to a limiting plateau. The first part represents a rapid reduction in heterogeneity driven by the bulk flow (convection); the slope of the RSD curve, in semi-logarithmic coordinates, is the convective mixing rate. The second part is driven by individual particle motion (dispersion) or by the slow erosion of API agglomerates due to shear.
Fig. 9.
Fig. 9. A typical mixing plot, with RSD plotted against number of revolutions. The two solid lines emphasize on the two distinctive mixing regimes.
When only one mixing mechanism is present (a situation that can be achieved by careful control of the initial loading pattern), a simple mass-transfer model, represented in Eq. (1) can be used, as in past studies [14], to capture the evolution of the RSD in powder systems. In this model, an exponential curve decaying towards a plateau is fitted to the mixing curves, where σ is the standard deviation, σ∞ the final standard deviation, A is an integration constant, λ signifies t
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