Gravimetric Feeding in Chemical Processing

A chemical plant feeds a wide variety of powders into reactors, dryers and blending vessels, and the list usually includes something hostile: a corrosive salt, an abrasive mineral filler, a hygroscopic powder that cakes on contact with humid air, or a product that is harmful enough that leakage is an exposure event.

Compatibility therefore comes before accuracy, and not only for the obvious parts. Screws, hopper walls, flexible sections, shaft seals and the lubricant all see the material, and so does whatever is used to clean between campaigns.

The bulk density range is the second recurring problem. Many chemical powders change density by a large factor between the aerated state after conveying and the settled state after standing, and a volumetric device cannot cope with that at all. Gravimetric control can, but only inside the turndown of the discharge device.

Then there is the installation. Feeders in chemical plants are frequently connected to pressurised or evacuated equipment, and a pressure difference across the feeder outlet applies a force the load cells report as mass.

A feeder whose wetted parts are verified against every product and cleaning agent in the campaign plan, at temperature, with wear parts identified and stocked.

Accuracy held across the full bulk density range, with the discharge device sized so the required turndown sits inside its usable range.

A weighing installation free of transmitted pipe forces and pressure effects, and an assembly certified for the area it stands in.

A cosmetics line is decided by the smallest pigment rate and by shade tolerance, where a feeder sized to look proportionate to the mixer is already wrong.

The compatibility matrix is built first: products, temperatures, cleaning agents, and every wetted component including seals and lubricants. This step frequently decides the machine, because the choice between a stainless screw, a coated one and a polymer lined hopper is made here rather than on performance.

The density range is then established by measurement, aerated and settled, because that range sets the required turndown. Where it is wide, the answer is often a larger screw turning slower rather than a smaller one at the top of its range, and sometimes two feeders rather than one.

The outlet connection is designed for the pressure it will see. A feeder discharging into a vessel under vacuum or under pressure needs a vent or a pressure compensation arrangement, otherwise the differential across the flexible connection appears directly in the weight signal and drifts with process conditions.

Area classification is applied to the complete assembly: drive, load cells, junction boxes and local control. And where the powder is harmful, the refill and cleaning operations are designed as containment tasks with extraction at the point of work.

• Compatibility matrix across products, temperatures and cleaning agents, for every wetted part.
• Bulk density range measured aerated and settled, and used to set the required turndown.
• Discharge device sized so the duty sits inside its usable range, not at the top of it.
• Outlet vented or pressure compensated where the receiving vessel is not at ambient.
• Complete assembly certified for the area, instruments and junction boxes included.
• Refill and cleaning designed as containment tasks where the powder is harmful.

• Chemical compatibility of screws, hopper, seals and lubricants with products and cleaning agents.
• Bulk density range between aerated and settled, which sets the turndown requirement.
• Abrasiveness, which decides wear parts and the interval at which the feed factor shifts.
• Receiving vessel pressure, which decides whether venting or compensation is needed.
• Area classification for the whole assembly.
• Exposure limit of the powders, which sets the containment requirement at refill and cleaning.