Using a Gravimetric Feeder to Dose Pharmaceutical Bulk Solids

Bulk-solids feeders are devices that meter the flow of bulk solids (e.g., powders) from a source (e.g., storage hoppers and intermediate bulk containers) to the downstream process at a precise flow rate. The optimal feeder depends on the powder's flow characteristics as well as the precision and throughput needed. In the pharmaceutical industry, single or twin-screw, volumetric or gravimetric feeders are commonly used to meter powders in various processes, such as milling, granulation, coating, direct compression, and blending. Pharmaceutical Technology spoke to Sharon Nowak, global business development manager for food and pharmaceutical, K-Tron, to find out more about how gravimetric, loss-in-weight feeders can be used in pharmaceutical bulk-solids processing, including their use in continuous-manufacturing processes.

Comparing volumetric and gravimetric feeders

PharmTech: How do volumetric and gravimetric feeders differ?

Nowak: Volumetric feeders control flow by metering a constant volume per time by regulating the speed of the feeding device. In the case of screw feeders, for example, this control would include setting the screw speed. The required speed is calibrated by weighing a timed sample. It should be noted that although there is no feedback to ensure feeding accuracy over time, this function may not be a concern for certain applications or materials with consistent bulk density. For this reason, volumetric feeders can be an economical choice for free-flowing materials and batch processes that require a lower degree of accuracy.

Gravimetric feeders, on the other hand, are real-time devices that meter the rate at a constant weight per unit of time. Weight is measured using a load cell; a feedback loop regulates the speed of the feeding device to control the feeder's accuracy. Gravimetric feeders as provided by K-Tron, for example, provide a much higher degree of accuracy, typically in the range of 0.25–0.5% of the required massflow setpoint.

How a gravimetric feeder operates

PharmTech: Can you explain further how a gravimetric feeder works?

Nowak: The most popular type of gravimetric feeder used in continuous processes is the loss-in-weight feeder (see Figure 1). Loss-in-weight feeders directly measure and control the process variable of flow rate and can fully contain the material within the confines of the feeder. Loss-in-weight feeders are typically either mounted on weigh scales or suspended from load cells. The K-Tron load cell, for example, is designed specifically for the rate and accuracy requirements of dynamic feeding and includes a resolution as high as 1:4,000,000.

Figure 1: A loss-in-weight gravimetric feeder uses load cells to continuously weigh the system. FIGURES COURTESY OF K-TRON

A loss-in-weight feeder consists of a hopper and feeder that are isolated from the process, so the entire system can be continuously weighed. As the feeder discharges material, system weight declines. The speed of the metering device is controlled to result in a per-unit-time loss of system weight equal to the desired feed rate. A typical loss-in-weight feeder controller adjusts feeder speed to produce a rate of weight loss equal to the desired feed-rate setpoint.

Any changes in material bulk density are sensed and accounted for by a change in metering speed. Precision, digital load cells from K-Tron, for example, use vibrating-wire weighing technology and digital filtering to discriminate between weight data and environmental effects, such as temperature, ambient vibration, and shock (Smart Force Transducer, K-Tron).

Measuring feeder accuracy

PharmTech: How is the accuracy of a gravimetric feeder determined?

Nowak: Weigh-feeder accuracy, regardless of the feeder type or design, is measured by weighing a series of timed catch-samples of material discharged from the feeder. The term 'weigh-feeder accuracy' refers to the combined effect of two distinct, but related, performance factors: linearity and repeatability.

Linearity, as the word implies, is a measure of the feeder's ability to deliver, on the average, the desired flow rate throughout the feeder's full range of operation (see Figure 2). A linearity measurement, therefore, reveals the difference between the actual and desired average sample weight at various flow settings. Repeatability, on the other hand, is a measure of the degree to which the feeder discharges a constant flow of material over a specified time period at a given flow-rate setting. Usually made at the intended nominal operating flow rate, a repeatability measurement indicates the level of scatter or dispersion (around the average sample weight) of the group of weighed catch samples.

Figure 2: Linearity measures the feeder’s ability to deliver a desired flow rate throughout the feeder’s range of operation.

A feeder's linearity measurement quantifies how well or poorly it delivers the desired average rate at each of various points throughout the feeder's complete operating range. Perfect linearity is represented by a straight-line relationship between the setpoint and the actual average feed rate throughout the feeder's specified turndown range from its design, full-scale operating range.

In addition, feeders should be designed to maintain accuracy during the refill phase, in which material is replenished in the feed hopper and the feeder is momentarily not being controlled by loss in weight. In the past, feeders were operated by a constant metering speed during refill, but because the bulk density of the material can change during refill, this often led to overfeeding. A more accurate method (Smart Refill Technology, K-Tron) stores trending data of the weight-to-speed ratio obtained while the hopper is emptying and uses this data to gradually change metering speed during refill. The speed correction allows the mass flow to remain constant during refill. Material characteristics (e.g., bulk density, particle size and shape, angle of repose, and gas permeability) and the refill hopper size also affect the refill process and its accuracy.

Using gravimetric feeders in continuous manufacturing

PharmTech: Continuous manufacturing is predicted to grow in use for solid-dosage manufacturing. What technology is needed to feed continuously?

Nowak: Gravimetric feeders are typically the technology of choice for continuous pharmaceutical processing, such as hot-melt extrusion or continuous direct compression, because the loss-in-weight controller is a real-time device that provides the accuracy needed for continuous process control. In a continuous process, the feeder sets the precise throughput for the downstream equipment, and feeding performance largely affects the performance of subsequent unit operations.

Due to the shorter residence times in continuous pharmaceutical processes, automatic sampling of feeder performance is often performed at smaller time intervals, from 15 seconds down to 5-second and even 1-second sampling. For this reason, it is imperative that the control system of the feeder chosen for continuous operations has fast response times. Although use of gravimetric feeders for continuous processing in the pharmaceutical industry is fairly new, these feeders have been an integral part of continuous processing in the food and plastics industries for decades.

Gravimetric feeders in tablet-press lubrication

PharmTech: How are gravimetric feeders used in tablet-press lubrication?

Nowak: Recently, gravimetric feeders have been used for direct, external lubrication of tablet presses, in which magnesium-stearate lubricant is blown into the press, which can reduce stearate use by as much as 97%. This significant reduction in the amount of lubricant added in the blending stage can drastically improve the blend properties, making it more free flowing as well as reducing the possibility of the formulation sticking to the tablet-press tooling. This concept was illustrated in a poster presented at an AAPS (American Association of Pharmaceutical Scientists) meeting (1). The use of a loss-in-weight feeder to control the rate of lubricant to the press allows processors to precisely measure how much is going in; by measuring how much stearate remains after processing, the amount of stearate in the formulation can be quantified (1). Typical feed rates of magnesium stearate for this application are 0.2–2 kg/h, and microfeeders allow rates as low as 50 g/h.

Reference

1. J. Nelson, S. Bell, M. Roy, J. Chu, and K. Waterman "Consistency of Magnesium Stearate Content Using External Lubrication in Tablet Compression," poster presentation at AAPS Annual Meeting & Exposition (Atlanta, Georgia, 2008).