Using a Gravimetric Feeder for Dosing 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, 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.

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 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 assure feeding accuracy over time, this 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.

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.

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 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).