Solids handling is not a unit operation. Therefore, it’s not covered in engineering courses. This leaves process engineers struggling to understand the “flowability” of bulk solids. This blind spot is huge. So, let’s talk about dryer selection and bulk solids handling.
Recently in The Chemical Engineer, Grant Wellwood described the handling process as the biggest industrial activity on the planet. The article estimated “that >70% of everything we use or consume involves bulk solids handling somewhere in its lifecycle.”
Mishandled, this process can quickly and efficiently destroy product value, careers, projects and even organizations. Yet, bulk solids flow is often an afterthought once the separation and drying equipment is selected. This article aims to bring bulk solids handling to the forefront.
Bulk Solids Handling Parameters
Bulk solids are defined as materials (solids) handled in various volumes and counts. Their flowability is impacted or controlled by friction (particle-particle or particle-surface). During the drying process, solids go through different phases such as free moisture, bound moisture, thixotropic and finally (and hopefully) free flowing.
The selected dryer must be able to handle each phase without creating fines, balls that can trap liquids, and without adding additional heat due to friction.
Here are some of the process and design parameters engineers need to consider for dryer selection:
- Dryer Process: Batch, Continuous, Atmospheric/ Vacuum, Turbulent, Gentle, Ring-Layer, Feeding (Volumetric or Gravimetric), Upstream and Downstream Equipment
- Recipes: Number of ingredients, Frequency of campaigns, Cleaning operations, Product integrity (fines generation) after drying and Residence time
- Dryer Performance: Batch size, Filling levels, and Production volume
- Product Characteristics: Quality, Bulk density, Tendency of segregation & agglomeration, Thixotropic phase, Shape, Size, Homogeneity, Risk of separation, Flow properties, Abrasiveness, and Moisture & Temperature
- Mixer design: Material of construction, Surface quality, Heating/cooling, Liquid feeding, Type of mixing tools, Speed of mixing tools and degree of back mixing
- Dryer Integration: Material flow, Physical space, Process sampling, safety requirements, etc.
It’s a lot to think about. Westwood observed in his thorough article, “When handling bulk solids, it’s always important to take a holistic or systems view because of the complex dependencies.”
BHS & Dryer Design
As my readers know, BHS provides for thin-cake filtration, cake washing and dewatering based upon pressure or vacuum, for batch or continuous operations from high solids slurries to clarification applications with solids to 1% and trace amounts.
In 2018, BHS acquired AVA mixers and dryers based in Herrsching (Munich) Germany. VA is in the unique position to provide both vertical and horizontal technologies providing for turbulent as well as gentle mixing, reacting and drying of wet cakes, powders and process slurries. The technologies are vacuum or atmospheric, batch and continuous, for final drying to “bone-dry” powders. The BHS technical article, Dryer Selection, explains the designs as well as selection parameters.
We know that solids change when processed from a wet-cake to bone-dry powder. Process engineers need to do the tests and trial and error to better understand these changes. As I often say, we can’t jump to conclusions.
Our process engineers would be happy to help at the BHS test center. With an understanding of how the flow properties change, depending on “complex interactions between particle size and distribution, moisture content and distribution, process history (time and manner), mineral composition, surface texture and condition as well as ambient conditions, just to name a few…” the dryer selection can begin in an educated manner.
Good luck and feel free to contact me for help with your questions.