Improving Powder Flow During Pharmaceutical Operations

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Manufacturing solid dosage forms such as tablets and capsules involves several powder handling steps, including blending, transfer, storage, and feeding to a press or a dosator.  The inability to achieve reliable powder flow during these steps can have a significant adverse effect on the manufacture and release of a product to market.  Production costs can be significantly higher than anticipated due to intervention required on the part of operators, low yield or unplanned process redesign.  An understanding of the fundamentals of powder flow can steer one in the right direction to achieve reliable powder flow.  Let’s begin by understanding what powder flowability really means.
Powder flowability
A simple definition of powder flowability is the ability of a powder to flow.  By this definition, flowability is sometimes thought of as a one-dimensional characteristic of a powder, whereby powders can be ranked on a sliding scale from “free-flowing” to “non-flowing”.  Unfortunately, this simplistic view lacks science and understanding sufficient to address common problems encountered by formulators, equipment designers and production personnel.
Anyone who has worked with powders, whether in a lab or a production setup, quickly recognizes that powder flow is complex.  Flow behavior is multi-dimensional and does in fact depend on many powder characteristics.  As such, no one test could ever quantify flowability.  In fact, flowability is not an inherent material property at all!  Flowability is the result of a combination of material physical properties that affect flow, AND the equipment used for handling, storing, or processing the material.  Equal consideration must be given to both the material characteristics and the equipment.  The same powder may flow well in one hopper but poorly in another; likewise, a given hopper may handle one powder well but cause another powder to hang-up.  Therefore, a more accurate definition of powder flowability is the ability of a powder to flow in a desired manner in a specific piece of equipment.
The specific properties of a powder that affect its flow are known as flow properties.  Examples of flow properties include bulk density, permeability, cohesive strength, and wall friction.  These flow properties arise from the collective forces acting on individual particles, such as van der Waals, electrostatic, surface tension, interlocking, friction, etc.  Although beyond the scope of this article, substantial amount of literature [1-3] is available on measurement of flow properties.
Flow patterns and flow problems
When a powder flows out of a hopper, bin or a container, one of two flow patterns will develop: funnel flow or mass flow.  Funnel flow occurs when the hopper walls are too shallow and too frictional for material to slide along them (see Figure 2).  As a result, the material along the walls remains stagnant, while material flows preferentially through a funnel-shaped channel directly above the outlet.  If the material is cohesive, arching and ratholing are common occurrences, resulting in no flow or erratic flow.  Fine powders may fluidize as they fall through a rathole, which may cause flooding and significant bulk density variations.  Funnel flow results in a first-in-last-out flow sequence which often leads to particle segregation [3] or exacerbates it.
Funnel flow is suitable only for sufficiently coarse, free-flowing powders, where segregation is not important.  This rules out the vast majority of pharmaceutical products.
Mass flow, the alternative to funnel flow, is characterized by a first-in-first-out flow sequence in which all of the material is in motion whenever any is withdrawn from the container (Figure 1).  This eliminates ratholing and provides a reliable discharge.  Mass flow generally minimizes segregation.  It also provides uniform bulk density during discharge.  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 [4].
To achieve mass flow, the hopper walls must be sufficiently smooth (having low friction) and steep to allow the material to slide along them.  In addition, the outlet must be large enough to prevent a stable arch from forming.  Both the required hopper angle and the minimum outlet size can be determined by analyzing flow properties test results.
An understanding of flow patterns provides a valuable insight in analyzing flow problems and determining solutions.  Each flow pattern has its own criteria, which can also be used to assess the suitability of existing equipment for handling a new material.  These evaluations can be used for blenders, portable bins, transfer chutes and feed hoppers.
Design options: reliable funnel flow design
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 [3].
  2. The powder has relatively low cohesive strength.  High cohesive strength may result in 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 an encapsulator, which may adversely affect the tablet or capsule properties or become a further source of segregation [4].
  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 even though consistent weights are required.  A funnel flow feed system will result in a more variable powder density than mass flow.

If all four of these conditions are met, funnel flow can be considered.  For a reliable funnel flow discharge, following requirements must be satisfied [5].
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].  Several equivalent changes can be considered to reduce the potential for ratholing.  The outlet size can be increased (within reason, of course, while still allowing it to interface with the downstream equipment), the powder fill height or equipment capacity can be reduced (thus reducing the consolidation of the powder and hence reducing the strength of the material that can give rise to ratholing), or custom equipment features such as agitation and/or mechanical assistance can be utilized.  If the option is available, changing (or reformulating) the powder to reduce its cohesive strength can also reduce likelihood of ratholing.  Common methods to reduce cohesive strength include increasing particle size, lowering the moisture content, and using a glidant.
If any 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 not practical for a given application, mass flow must be utilized.  
Design options: mass flow design
Mass flow discharge from a bin, portable container or a hopper occurs when the following two design criteria are meet:

  1. The sloping walls are smooth and/or steep enough to promote flow along the walls;
  2. The bin outlet is large enough to prevent an arch.

Following techniques must be used while considering a mass flow discharge [5].  These techniques may be applied to designing a new equipment or modifying an existing equipment to provide mass flow.

  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 4) determined via cohesive strength testing.  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 [2,4].  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 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 (qc), based on wall friction tests.  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 qc.  Planar walls should be equal to or steeper than the recommended mass flow angle for a planar hopper (qp), 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 (qc/qp).
  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 (qc/qp) 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.
References

  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. 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.
  4. Baxter, Thomas J., “When Powders Flow Like Water:  Addressing Two-Phase Flow Effects in Tablet Press Feed System”, Tablets & Capsules, March 2009, Volume 7, No. 2, pp. 26-32.
  5. Barnum, Roger, Ebb and Flow: Understanding Powder Flow Behavior, Pharmaceutical Processing, March 2009, pp. 18-21.

Figure 1:  Manufacturing pharmaceutical tablets and capsules involves several powder handling steps.

Figure 2:  The two primary flow patterns that occur in gravity discharge are funnel flow and mass flow.

Figure 3:  Stagnant, cohesive powders can form a stable rathole in funnel flow.

Figure 4:  Critical parameters to select when designing mass flow bins and hoppers

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