Selecting the Right Type of Filtration for Solid-Liquid Separation

solid-liquid separation
Photo by Picturepest on / CC BY

Filtration selection, if we think back to Sherlock Holmes, means “not jumping to conclusions.”  There is no “one size fits all” process solution.  Selecting a filtration technology requires a systems approach incorporated with other solids processing such as reactors, dryers, solids handling, etc.  You could gain an objective overview by filling out an application data sheet (like the ones I use for new or existing applications) that can help identify what’s involved in the specific solid-liquid separation.

Ultimately, the process has three components:

  • Material properties, which I’ll describe in more detail below
  • Separation performance objectives including, for example, filtrate quality (conductivity or residual solids) cake dryness, flowability of the cake, crystal breakage /fines generation and conditioning of the cake for further processing
  • Mechanical properties — The specification must be clear in terms of material of construction, temperatures/pressures, FDA validation, cleaning procedures, manufacturing codes, etc.  Each equipment type will have its own mechanical specifications that must be satisfied.

These three considerations are combined and ranked choices are then evaluated for operational, economic, and plant (internal and external) objectives.

Finding the Best Filtration Procedure

Your examination of material properties considers the solids and the liquids.  For solids, the engineer needs to know the total suspended solids (TSS) and solids concentration, particle size distribution (PSD), and particle shape.  The PSD should be based upon particle counts at different sizes rather than by weight or volume.

The particle shapes can vary:  spheres, rounded, angular, flaky, or thinly-flaked are among the examples.  Shape will influence the filtration rates for the process and also impact the PSD due to the nature of particle size measuring equipment.

Knowing this, the solid-liquid filtration system further requires a systems approach to incorporate other solids processing such as reactors, dryers, and solids handling, etc.  The full scope should include the actual upstream and downstream.

Consider this typical example of a chemical process including all of the associated processing steps:

  • Chemical synthesis and Crystallization:
    • Types of catalysts
    • Solvents
    • Continuous or batch
    • Temperature
    • Flashing
    • Inerting
  • Filtration
  • Drying
  • Dissolution
  • Hydrogenation
  • Secondary crystallization
  • Filtration
  • Final drying
  • Solids and slurry handling in all steps

General Guidelines to Selection

So, the question is where to begin to make the preliminary filtration technology choices for solid-liquid separation?  Here are some general guidelines to selection:

Filter Press Continuous Vacuum and Pressure Nutsche  Filter & Filter-Dryer Clarification
Solid content of the suspension (%) 5 to 30 10 to 40 10 – 40 < 5
Maximum Pressure Difference 100 bar -1 to 6 bar 6 bar 10 bar
Cake Thickness (mm) 5 to 50 5 to 150 5 to 300 20
Average Particle Size 1 to 100 micron 1 to 100 micron 5 to 200 micron 1 to 50 micron
Type of Operation Batch Continuous Batch Batch
Comments Good for slow filtration and can produce dry filter cakes; Excellent cake washing and pre-drying Good when reactor batch times equal to total cycle times Disposable for low flows; candle and plate filters for large flows

Let me know if this is helpful to you.  My idea is to do a series of types of filtration systems for solid-liquid separation for various applications.  What is troubling you?