When designing a continuous process complete with gravimetric loss-in-weight feeding, it's imperative that the refill device be engineered for feeder accuracy and process quality. Pneumatic conveying receivers can ensure dust-free, efficient transport and refill. Here's how…
By Sharon Nowak
Continuous processing has long been a mode of manufacture for many industries including petrochemical/chemical, plastics, and food due to its advantages in control of product quality and overall manufacturing costs. Even the pharmaceutical industry, a long time user of batch processing, is currently actively investigating the use of continuous processing for milling/micronization, mixing, granulating, extrusion, and drying due to its inherent advantages. Regardless of the exact continuous process utilized, the method of feed and refill to the process is critical in maintaining process quality and efficiency. This article will explore the general principles of feeder refill with a focus on pneumatic conveying as a preferred method of this refill.
The mode of refill to the loss-in-weight feeder that is feeding continuous processes such as mills, blenders, granulators, and extruders can be almost as critical as choosing the right feeder technology. The speed of the feeding device is controlled to result in a per-unit-time loss of system weight equal to the feed rate. As an integral part of loss-in-weight feeding, the weighed hopper must be resupplied with material. To continuously resupply the hopper would destroy the very basis for control or, rather, the decline of the weight value. Hence, the hopper must be resupplied periodically and quickly rather than continuously.
When refilling a continuous process, the refill devices must reliably maintain active flow of the ingredients to the process within specific refill time limits. This time limit must be relatively short in order to allow the feeder to return to a true gravimetric operation and ensure constant mass flow of the product into the process. Additionally, the cut-off action of the selected device must be quick and sure. A slow tapering off of the refill needlessly lengthens refill time. Any leakage of the refill device may cause a weight disturbance, resulting in a flow error in the positive direction.
Refilling MethodsTwo basic ways exist to refill the feeder hopper: the manual method and the automatic method. The manual refill method implies that a quantity of bulk solids is tossed into the feeder hopper by the plant operator, and then the process continues. The automatic refill method implies that dosing machinery under control of the feed system will add material to the feeder hopper from an upstream supply.
Traditionally, a constant metering speed is maintained throughout the refill phase a speed corresponding to the metering speed associated with gravimetric control just prior to entering the refill phase. If, for example, the metering speed averaged 60 rpm just prior to the system sensing the need to refill the supply hopper, the screw speed would be maintained at that 60 rpm for the duration of the refill operation. After refill is completed, material has settled, and the feeder senses an appropriately declining system weight, the feeder is returned to gravimetric operation where metering speed once again becomes the parameter of control.
Acceptable Interval?What refill interval is deemed acceptable? As a rule, the refill period should be about 6-10 seconds. This duration ensures positive control over the incoming material but is so short that minor flow variations should not perturb the downstream processes.
Specific material characteristics will greatly affect not only the flow out of the hopper but also how quickly the hopper is refilled. For example, if the bulk material has a high gas permeability (i.e., if gas can escape rapidly from the solid), the refill can occur as rapidly as is feasible. On the other hand, if the bulk material has a very low permeability rate (i.e., the gas expands the bulk material and doesn't quickly escape to return the solid to its resting condition), care must be taken during refill because these materials often become floodable in this condition or at least suffer significant bulk density changes.
The reason for this concern is simple. When the feeder hopper is empty of bulk solids, it is actually full of air. If the refill occurs quickly with a bulk solid that suffers from entrained air problems, displaced air flowing back through the bulk solid to the vessel above may become entrained air in the solid.
If flooding of the bulk solid begins during refill, the product could easily flush out of the loss-in-weight feeder. At a minimum, air entrainment effects resulting in underfeed conditions during and immediately after refill can significantly alter bulk density, since the control system is using control data from earlier in the run, when the product density was much higher. Air entrainment comes from rapidly refilling the feed hopper such that air, being unable to quickly escape, passes through the incoming material to the region of lower pressure above in the refill hopper. Venting of the weighed hopper can minimize the problem, but now dust collection and, in the case of active materials, filtration must be arranged. Adequate venting will assist in material settling and aid in a quicker return to true gravimetric operation.
Generally, the maximum refill time should not exceed 10 percent of the total feed/refill cycle time. This consideration is largely irrelevant in low rate systems where cycle times are long and where hoppers are small and can be refilled quickly. However, in many intermediate to high rate systems, this 10 percent limit will dictate the required delivery rate of the refill device.
Gravimetric blender with vacuum receivers above each feeder to provide refill
For example, in refilling a feeder hopper from an intermediate bulk container, super sack, or large hopper, the volume of the product in these vessels will often exceed the volume of the feeder hopper. Clearly, it is impossible to control the refill without overfilling the feeder hopper unless either a time window is established for the opening and closing cycle of the refill valve or the use of a pneumatic receiver is used as a buffer hopper between the intermediate bulk container and the feeder.
As stated previously, the flow rate from the refill device must be sufficient to avoid exceeding the 10 percent refill time limit. Additionally, the flow cutoff action of the selected device must be quick and sure. A slow tapering off of the refill flow needlessly lengthens refill time, and any leakage of the refill device may cause an unavoidable measurable weight disturbance but will always result in a flow error in the positive direction.
Pneumatic ReceiversAs mentioned above, pneumatic receivers that operate under a dilute phase vacuum transfer principle are often used as refill devices, particularly for continuous operations. The pneumatic system uses negative pressure via a vacuum to suck the material required to refill into a separate mounted and supported vacuum receiver. The receiver is filled to a determined level and then holds this material charge until the feeder below requests a refill. The level of fill in the receiver is determined by level sensors. At the point of refill request by the feeder below, the discharge valve opens and the contents are discharged into the feeder hopper. For loss-in-weight feeding, this discharge valve is usually a pneumatic butterfly valve in order to prevent any chance of dribbling. At the same time of material release, a gas pulse is sent through the filter housed in the vacuum receiver in order to release any entrained particulate or material that may have settled on the filter. The filter material can vary. Options include laminated membrane type materials for quick release and easy clean properties.
After dumping the material into the feeder hopper below, the valve is shut. The receiver vacuum cycle immediately begins again in order to be ready for the next refill request.
Sharon Nowak is the global business development manager with K-Tron Process Group, Pitman, NJ. Additional information is available by calling 856-589-0500 or by visiting www.ktron.com.