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Coalescence

The challenge: Reaching superior performance in liquid-liquid or liquid-gas separation applications

Understanding coalescence in filtration processes

The development of effective filters requires a deep understanding of the physical and chemical processes playing a role in the behaviour of liquids. A particular focus is on coalescence - the process by which smaller liquid droplets fuse together to form larger droplets. This dynamic is critical to the function and efficiency of many types of filters, especially in applications such as water treatment, oil-water separation and air purification.

Fig. 1 Coalescence of droplets in an emulsion

Coalescence in filtration applications

Controlling the coalescing process is crucial in industries that involve machining, metalworking, and manufacturing and where oil mist from cutting, grinding, or lubricating processes may pose a health risk to workers and pollute the environment.

Coalescer filters typically consist of specialized media or material with high surface area and specific properties to facilitate the coalescence process efficiently. These filters are commonly used in various applications:

  • Removing contaminants such as water, oil mist, and other liquids from gas streams, thereby improving product quality
  • Protecting equipment from damage or corrosion
  • Ensuring compliance with environmental regulations

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Video 1: Coalescence on a filter media

Simulation of coalescence at the filter media scale

GeoDict uses a three-step approach in simulating coalescence at the media scale

  1. Flow and pressure field calculation: Flow and pressure field are calculated solving the Navier-Stokes-Equation, using state-of-the-art numerical solvers and techniques such as adaptive grid, multigrid, and Krylov sub-space.
  2. Agglomeration of particles: Particles are tracked along the streamlines, which are influenced by drag, diffusion, and electrostatic forces. After all particles are filtered (or have left the medium), the particles become part of the structure – concluding a so-called batch. In particular, for the simulation of coalescence, particles become fluidized and can agglomerate to larger droplets.
  3. Flow field recalculation: For the next batch, the flow field is recalculated and a new set of particles is tracked. By running multiple batches, the depth of the filter media gets clogged and a filter cake can form.

The shape of the droplets adherent to the fiber surface is governed by the contact angle between the fiber and droplet. 

Other coalescence-related processes can also be specified and included in the simulation, such as re-entrainment and drainage. Re-entrainment refers to the phenomenon where liquid droplets, after being captured by a coalescer filter, re-enter the gas or vapor stream.

 

Video 2: Particle Deposition Coalescence

Challenges in developing the coalescer filters of the future

Fiber diameter and shape

Promote effective coalescence by optimizing the diameter and shape of fibers in the filter. Finer fibers can capture smaller liquid droplets, while a controlled pore structure enhances liquid retention and facilitates droplet merging. Multi-layered or gradient-density media configurations may further improve coalescing efficiency by providing multiple stages of droplet capture and coalescence.

 

Modification of fiber surface

Improve droplet wetting and coalescence by modifying the surface properties of filter media through treatments such as hydrophilic or hydrophobic coatings.

Surface modifications can alter the surface energy and affinity for specific fluids to help trap and bind targeted contaminants.

Pore size distribution

Capture a wide range of droplet sizes effectively by tailoring the pore size distribution of the filter media.

A balanced pore size distribution, including both micro- and macro-pores, ensures efficient removal of both small and large droplets from the gas or vapor stream.

Media thickness and density

Achieve the desired balance between coalescing efficiency and pressure drop by optimizing the thickness and density of the filter media. Thicker media with higher density captures more liquid droplets, but may increase pressure drop and require more frequent maintenance. Balancing these factors is crucial to maximize filtration performance while minimizing operational costs.

The GeoDict solution for coalescence in filter applications

The GeoDict package includes, in addition to GeoDict Base, all the modules needed for research and development of filter applications.

Module Recommendations

Image Processing & Image Analysis ImportGeo-Vol  
Analysis & Characterization FiberFind(-AI) PoroDict + MatDict
Modeling & Design FiberGeo WeaveGeo
Simulation & Prediction FilterDict FlowDict

Suitable modules may vary and depend on the specific application.

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