Bulk Material Feeding in Continuous Processes

Many of today's sophisticated continuous processes could not function without high-accuracy, highly reliable feeding systems

In a continuous process, all equipment components related to the required unit operations function as a single system and are designed to operate 24 / 7, except for periods of required maintenance. This increased equipment utilization and the economies of scale associated with continuous processing create a variety of cost savings and operations efficiencies.

Historically, many high-volume and commodity chemicals, petrochemicals, plastics, and mass-market foodstuffs have been produced using continuous processing. Increasingly, high-value, lower volume processes, as found for example in the pharmaceutical industry, are now going continuous as well.

Converting from batch to continuous operations

It can make good engineering and economic sense for process operators to convert existing batch processes to continuous operations.

• Increased throughput with equipment operating continuously under proven, steady-state conditions
• Smaller equipment components with smaller process footprints produce higher throughput volumes than larger batch components
• More consistent product quality with batch run-by-run variations eliminated
• Reduced energy use with multiple heating and cooling steps for each successive batch eliminated
• Reduced consumption of raw materials and production of off-spec product by eliminating batching’s successive ramp up and ramp down
• Reduced manual handling of raw materials and finished product
• Better containment of dust and airborne emissions thanks to the enclosed nature of most continuous processes
• Save Time and costs by eliminating batching tasks such as staging and loading materials for each batch, sampling and discharging the completed batch, inventory control, and cleaning reactors, vessels and other components between successive batches

Often facilities that convert from batch to continuous produce the same volume of product in a matter of days that once took weeks or months to produce.

Feeder options for continuous processes

Either volumetric feeders or gravimetric feeders can be used to deliver bulk solids into a continuous process, depending on the nature of the materials and the needs of the process.

Volumetric feeders for continuous processing: Volumetric feeders are employed in continuous processes where the bulk density of the material fed is consistent or where feeder accuracy is not a concern. They are essentially open-loop devices, whose final discharge rate is a function of constant feeder speed and is not governed by any ongoing data feedback from the operation.

Since these systems cannot detect or adjust to variations in the material’s bulk density, the actual amount of material delivered to the process (the mass flow) can vary over time. This makes volumetric feeders inappropriate for continuous process applications that have rigorous accuracy requirements.

Gravimetric feeders for continuous processing: The two main types of gravimetric or weigh feeders are loss-in-weight feeders and weigh-belt feeders. Thanks to their high repeatability and ability to capture and document moment-by-moment feeder performance during continuous process operations, gravimetric feeders are widely used during continuous processing at many CPI facilities.

Weigh feeding systems provide precise discharge rates and an immediate alert if there is any interruption of material flow. Realtime gravimetric feeding data supports the feedback loops needed to achieve sophisticated process control. Both realtime and historic data provide a useful window onto the process. These analytics support troubleshooting efforts, allow for statistical quality-control analysis, and can help operators prevent downstream accuracy or quality problems.