Understanding Powder Flow Behavior
Manufacturing solid dosage forms involves several powder handling steps, including blending, transfer, storage, and feeding. This article discusses common powder handling problems, the powder and equipment properties that can influence these problems, and some possible solutions.
The inability to achieve reliable powder flow can have an adverse effect on the manufacture and release of a product to market. Production costs can be higher than anticipated due to excessive intervention by operators, low yield or unplanned process redesign. An understanding of key flow properties and equipment parameters can reduce these risks. Although beyond the scope of this article, there is a substantial amount of available literature regarding the measurement of flow properties1-5 and addressing uniformity concerns6, 7 that can be used to further understanding.
A powder (or bulk solid) is simply a collection of discrete solid particles. The concepts discussed in this article apply to many types of solids including individual ingredients, granulations and dry blends. Flowability describes the ability of a powder to flow through equipment reliably. There is a tendency to define flowability as a one-dimensional characteristic (e.g., a single "flow property"), usually ranking the powder on a scale from free flowing to non-flowing. A single parameter, however, is insufficient to fully define a powder's handling characteristics or to provide design parameters needed to address common handling concerns. Powder flow behavior is multi-dimensional, and therefore a range of flow properties must be measured to characterize it 2.
The observed flow behavior of a given powder is a function of both its flow properties and the design of the equipment used with it. A poor-flowing powder can be handled reliably in properly designed equipment, while a good-flowing powder may develop problems in improperly designed equipment. Therefore, a more accurate definition of flowability is the ability of powder to flow in the desired manner in a specific piece of equipment.
One of the first steps in assessing potential flow problems is to determine the flow pattern that will occur during gravity discharge of the powder through the handling equipment. There are two primary flow patterns that can occur: funnel flow and mass flow (Figure 1). In funnel flow, an active flow channel forms above the outlet, which is surrounded by stagnant material. This is a first-in, last-out flow sequence. It generally occurs in equipment with relatively shallow hoppers, often including hopper geometries such as the asymmetric cones common to tablet press feed hoppers and bins with rectangular-to-round transitions (Figure 2). In funnel flow, as the level of powder decreases, stagnant powder may fall into the flow channel, provided the material is sufficiently free flowing. If the powder is cohesive, a stable "rathole" may develop (Figure 3).
In mass flow, all the powder within the equipment moves when any is withdrawn. Mass flow provides a first-in, first-out flow sequence, eliminates stagnant powder, and provides a steady discharge with a consistent bulk density. As a result, designing handling equipment for mass flow provides many benefits. Achieving mass flow requires an outlet large enough to prevent arching and hopper walls that are steep and smooth enough to allow the powder to flow along them. Flow properties tests and calculations are used to determine these design parameters 1.
Although mass flow designs can overcome a number of potential flow problems, it should be noted that adverse two-phase (powder and interstitial gas) flow effects can still remain. These effects can limit production rates and/or result in unacceptable variation in the product properties (weight, thickness, hardness, dissolution, etc.). This behavior is most common with fine powders (i.e., mean particle size of 150 microns or less)2.
An understanding of flow patterns is one of the steps that should be used in designing handling equipment. Each flow pattern has its own criteria, which can also be used to assess the suitability of existing equipment for a new material. These evaluations can be used for equipment ranging from storage silos to portable bins, transfer chutes and feed hoppers.
Reliable funnel flow design
Although mass flow has many advantages, there are instances where funnel flow is appropriate. A funnel flow design can be considered if all of the following conditions are met:
1. Segregation is not a concern. Since funnel flow will result in a first-in, last-out flow sequence, any side-to-side segregation that occurred when the equipment was filled will often be exacerbated 7. 2. The powder has relatively low cohesive strength. High cohesive strength may result in the formation of a stable rathole. 3. Flooding is not a concern. Flooding can result in a highly aerated (low-density) powder from a feed system to a tablet press or encapsulator, which may adversely affect the tablet or capsule properties or become a further source of segregation 2. 4. Bulk density variations are not a concern. Much of the machinery used to produce final dosage forms (e.g., tablets and capsules) relies on volumetric feeding. A funnel flow feed system will result a more variable powder density than mass flow, since the powder will be subjected to different consolidation pressures depending on where in the equipment it was recovered from.
If all four of these conditions are met, funnel flow can be considered.
The first requirement for such a design is the outlet diameter, which should be greater than the critical rathole diameter (Df) calculated from cohesive strength test results, to ensure that a stable rathole will not form 1. If ratholing is a concern, several changes can be considered to minimize the potential. The outlet size can be increased (within reason, of course, while still allowing an interface with downstream equipment), the powder fill height or equipment capacity can be reduced (thus reducing the consolidation of the powder within, and the resulting Df), or custom equipment features such as agitation and/or mechanical assistance can be utilized 8. If the option is available, changing (or reformulating) the powder to reduce its cohesive strength can also reduce the likelihood of ratholing. Common methods to improve this property include increasing particle size, lowering the moisture content, and using a lubricant or glidant.
If one of the conditions for selecting funnel flow is not met (e.g., segregation is a concern), or the options available to prevent a rathole are infeasible or impractical for a given application, mass flow should be utilized. A mass flow design cannot rathole because the powder moves along the hopper walls.
Designing for mass flow
When designing for mass flow, these general guidelines should be followed:
1. Size the outlet to prevent a cohesive arch. Cohesive strength test results are used to determine the required minimum sizes. The outlet diameter should be equal to or larger than the minimum diameter (Bc, Figure 5). If a slotted outlet is used, the outlet width should be sized to be equal to or larger than the minimum slot width (Bp), provided its length is at least 3 times its width. The outlet may also need to be sized based on feed rate and two-phase flow considerations 2. If the outlet cannot be sized to prevent an arch (e.g., because the tablet press hopper outlet must mate with a fixed feed frame inlet), agitation and/or mechanical assistance can again be considered.
2. Once the outlet is sized, design the hopper wall slope to be equal to or steeper than the recommended hopper angle for the selected wall surface. For a conical hopper, the walls should be equal to or steeper than the recommended mass flow angle for a conical hopper (?c), based on wall friction tests (Figure 5). If the hopper has a rectangular-to-round hopper, the valley angles (that form at the intersection of adjacent side walls) should be sloped to be equal to or steeper than ?c. Planar walls should be equal to or steeper than the recommended mass flow angle for a planar hopper (?p), provided the outlet length is at least 3 times its width and the feeder or valve below can provide active discharge over the entire outlet area.
3. Thoroughly specify interior wall surface finish. It is not sufficient to simply call for a type of stainless steel in a mass flow design without specifying the interior finish, because wall friction of the powder may vary significantly from one finish to another. It also cannot be assumed that a reduced average roughness (Ra) of the interior surface finish will allow for more shallow designs, because fine powders can become more frictional with reduced surface roughness due to increased particle/surface contact area. Therefore, measure the wall friction of the powder against the interior surface finish being considered to determine the required angles for mass flow (?c/?p).
4. Consider velocity gradients. Even when equipment is designed for mass flow, there will be a velocity gradient between material discharging at the hopper walls (moving slower) compared to its center (moving faster). An increase in the velocity gradient can be used to enhance blending between vertical layers of material, while a reduction can be used to enhance side-to-side mixing (e.g., to minimize the effects of segregation). Making the hopper slope steeper with respect to the recommended mass flow hopper angle (?c/?p) or changing the surface finish to reduce wall friction will reduce the velocity gradient. Asymmetric hoppers, which are common to tablet presses, are especially prone to velocity gradients because powder moves faster at the steeper hopper wall.
5. Avoid upward-facing lips or ledges. These often occur where flanges mismatch or where there are level probes, view ports, gaskets, or partially opened valves protruding into the flow path. Ideally, devices that protrude into the interior are installed in non-converging (vertical walled) sections of the equipment, where they are less likely to upset the mass flow pattern.
In conclusion, costly powder flow problems can be avoided with a measure of prevention: determine the flow properties of the powder for use in designing or selecting its handling equipment.
1. Jenike, A.W., Storage and Flow of Solids (Bulletin 123 of the Utah Engineering Experimental Station), 53 (26), (1964, revised 1980). 2. Prescott, J.K., and Barnum R.A., On powder flowability, Pharmaceutical Technology, October 2000, pp. 60-84 and 236. 3. Schulze, D., Measuring powder flowability: A comparison of test methods, part 1, Powder and Bulk Engineering, 10 (4), pp. 45-61 (1996). 4. Schulze, D., Measuring powder flowability: A comparison of test methods, part 2, Powder and Bulk Engineering, 10 (6), pp. 17-28 (1996). 5. Standard shear testing method for bulk solids using the Jenike shear cell, ASTM Standard D6128-97, American Society for Testing and Materials (1998). 6. Prescott, J.K. and Garcia, T. P., A solid dosage and blend content uniformity troubleshooting diagram, Pharmaceutical Technology, March 2001, pp. 68-79. 7. Prescott, J. K. and Hossfeld, R. J., Maintaining product uniformity and uninterrupted flow to direct compression tablet presses. Pharmaceutical Technology, 18 (6), 1994, pp. 99-114. 8. One such valve is available from Matcon, www.matconibc.com.