Shear cell equipment

Figure 5.1 Shear cell equipment 1) shearing zone, 2) heating elements, 3) rotating plate, 4) non-rotating cone, 5) thermocouple, 6) torque measurement point. Cone angle = 100°, shear zone angle = 2.5°, R = 0.1 m, H = 0.082 m [25].

The weaknesses associated with the flow through an orifice and angle of repose measurements limit their application for powder flow studies and hopper designs. Consequently, several powder shear testers and methods that permit a more thorough and precisely defined assessment of powder flow characteristics were developed. Shear testers that measure the frictional characteristics of a powder bed under load yield valuable information with regard to powder flow in high-speed tablet equipment. A number of types of shear cell testers are available, but the most common types used in the pharmaceutical industry are the Jenike shear cell and the Schulze ring shear tester.61,62... 

The so-called shear cells are used for direct shear tests, where the powder specimen is consolidated in the vertical direction and then sheared in a horizontal plane. There are basically two types of shear cells in use today the Jenike shear cell (sometimes referred to more generally as the translational shear box) and the annular (or ring) shear cell (the rotational shear box). As the equipment needed is highly specialized (and hence outside the scope of this Guide) and as manufacturers instructions are usually adequate, the following contains only an outline description of both the hardware and the test procedures. 

The shear cells as used in testing yield strength of solids may also be used for testing friability. As large strain is required in order to produce significant attrition, the annular shear cells (which permit infinite strain) are usually used. A variant on the annular shear cell for attrition testing is available commercially from Ajax Equipment in Manchester. 

The Jenike shear cell has been considered for long time the testing cell for establishing standard procedures in industrial applications and research. It has been recognized as one of the standards for testing bulk solids in the United States and in Europe, being especially focused on cohesive powders. The complexity of this method is such that errors due to poor technique can easily arise. A reference material has therefore been produced with which laboratories can verify both their equipment and experimental technique. The reference material consists of 3 kg of limestone powder packed in a polyethylene jar. It is accompanied by a certificate giving shear stress as a function of normal applied stress for four different powder compaction stresses. 

A rotational shear cell instrument, such as the FT4 Powder Rheometer, equipped with a 48 mm rotational shear cell and a 30 mL shear measurement vessel. A batch of reference limestone powder (CRM 116), produced and sold by the Commission of European Communities. 

Some bulk solids gain strength, when stored under pressure without movement. Principally this time consolidation can be tested with all testers. Besides the fact that time consolidation can most easily be tested with Jenikes tester and a new version of an annular shear cell [10] -easily with regard to time and equipment - only these testers yield Time Flow Functions which have to be known for applying the doming and piping criteria. 

Fig. 3 shows the experimental apparatus. The feed tank had a 50 gallon capacity and was equipped with a variable speed mixer. The feed pump was a flexible impeller, positive-displacement pump to minimize shearing of the feed emulsion. The pumping rate was regulated by a Graham Variable Speed Transmission. Each flotation tank was 11.5 in. ID with 6.5 in. liquid depth maintained by a CE IN-VAL-CO conductometric level controller with a pneumatically actuated control valve in the effluent line. The fourth cell was not equipped with an air inducer. The outer diameter of the air downcomers was 1.5 in. The rotor in each air inducer was a turbine taken from a 2 in. turbine flow meter. Each rotor was belt driven by a 10,000 rpm, 1/30 hp motor and all three motors were governed by the same variable transformer. Another pulley on each rotor shaft was attached to a non-powered belt connecting all three shafts to ensure that each rotor turned at the same speed. 

Many alternative fermenters have been proposed and tested. These fermenters were designed to improve either the disadvantages of the stirred tank fermenter-high power consumption and shear damage, or to meet a specific requirement of a certain fermentation process, such as better aeration, effective heat removal, cell separation or retention, immobilization of cells, the reduction of equipment and operating costs for inexpensive bulk products, and unusually large designs. 

Stirred- tank 1. Flexible and adaptable 2. Wide range of mixing intensity 3. Can handle high viscosity media 1. High power consumption 2. Damage shear sensitive cells 3. High equipment costs... 

The techniques that have been used to characterise the mechanical properties of microparticles may be classified as indirect and direct. The former includes measurement of breakage in a "shear" device, for example, a stirred vessel (Poncelet and Neufeld, 1989) or bubble column (Lu et ah, 1992). However, the results from these indirect techniques are rather difficult to use since the mechanical breakage depends not only on the mechanical properties but also the hydrodynamics of the processing equipment, and the latter are still not well understood. To overcome this problem, a cone and plate viscometer that can apply well-defined shear stresses has been used to study breakage of hybridomas (Born et ah, 1992), but this is not a widely applied or applicable technique because the forces are too small to break most cells. 

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