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 type of filtration is best 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 for selecting among types of filtration:
| 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?
With all the data on hand, it is better to do some bench scale filter testing in a small lab filter to determine the how well the product filters and what factors affect the filtration allowing consideration of the filter media to be used and what filter aids should be considered in filtering the product. These factors can be resolved with the bench scale testing, that is my first recommendation when faced this problem, this is my own personal recommendation besides what is discussed above which is very important