Getting Rid of Gel Particles What Is the Requirements Profile for a Gel Particle Filtration System?

Editor: Manja Wühr

Filtration systems can help alleviate process problems caused by gel particles. This article explains the requirements that these systems have to meet.

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Multi-layer stainless steel non-woven fiber material for depth filtration. (Pictures: Lenzing Technik)
Multi-layer stainless steel non-woven fiber material for depth filtration. (Pictures: Lenzing Technik)
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Finding lumps in your pudding is annoying. However, insoluble particles can cause more serious problems in the process industry. Gel particles can be present wherever solids are dissolved in a solvent, for example during the production of processed cheese, synthetic resins or biodiesel, resulting in product quality degradation or equipment malfunction. If the particles cannot be dissolved in the process, filtration systems can be deployed to remove them.

Key aspects to consider

The word gel is derived from gelatin, and it refers to a dispersed system with a dimensionally stable, easily deformed dispersed phase. This statement only applies to a certain extent to gel particles, because dimensional stability only holds true under certain conditions. Studies have shown that the consistency of gel particles can vary considerably, ranging from a rubbery consistency to particles which are still in the fluid state and merely have a higher viscosity than the surrounding fluid. Gel particles can have different properties, and that places significant demands on the filtration system.

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Depth filtration

It is hardly surprising that compared to screens, the material used for depth filtration is much more effective at removing soft particles. The reason is that a gel, which fits through a pore in the material, is captured in a deeper layer. In order to exploit this effect, Lenzing Technik uses a multi-layer, stainless steel, non-woven fiber fabric on its RKF and AKF series for backwash depth filtration applications.

Differential pressure

It is also easy to understand that if the particles are completely hard and the pores in the filter material are totally rigid, the differential pressure which acts on the particle in the filter material during the filtration process may increase infinitely without having a negative impact on the quality of the filtrate. For softer particles however, the differential pressure must be lower during filtration to prevent the particles from being forced through the pores in the filter material. The schematic representation in Fig. 1 shows how hard particles react compared to soft particles on the surface of the filter material.

Lenzing Technik uses extremely fine stainless steel fibers (as small as 2 µm) to produce a highly porous filter material. This feature, plus the absence of any pleating, makes the filtration systems suitable for very high viscosity media at differential pressures between one and three bar. In rare cases when the filters are in operation for several months, the differential pressure can rise to five bar.

Texture of the filter materials

Soft filter material, for example filter felt, has one major disadvantage. If the flow rate varies or particle loading increases, the differential pressure and the shape of the pores in the filter also change. The filter medium is compressed to a greater or lesser extent, and the pores either open wider or they become more constricted. As a result of these changes, forces acting on the gel particles cause them to deform and press them through the filter medium in the worst case scenario.

A particle, particularly a gel, obviously exerts a force on the surrounding material (e.g. the felt fibers) when a pore is blocked, eventually causing the particle to slip through. The schematic representation in Fig. 2 illustrates this effect.

On the RKF and AKF filters, the problem was solved by mounting a special stretching frame which holds the filter material over a perforated support tube The tension is adjusted using precisely defined clamping forces to ensure that the strain applied to the filter material by the clamping frame is equivalent to a differential pressure of six bar. If you now consider the balance of forces on the other side of the filter material, it is apparent that the perforated drum (support tube) exerts an equal force on the filter material in the opposite direction. As a result, the filter material is in a state of force equilibrium (Fig. 3).

If filtration is started from the inside out, the differential pressure across the filter medium increases. For example, let us assume that the pressure is two bar. To maintain the balance of forces, the differential pressure relieves the load on the perforated drum due to the direction of filtration. The new force equilibrium is as follows: from the one side, four bar applied by the perforated drum plus two bar of differential pressure equals six bar; from the other side, the tensioning cage continues to apply six bar (Fig. 4).

It is apparent that, up to the level of pre-tension applied by the tensioning frame (6 bar), the filter material retains its structure, thickness and pore size even if the differential pressure changes. This design, in combination with the natural rigidity of sintered metal fiber non-woven fabric, produces an extremely rigid structure.

Gel particle residence time in the filter

Several reference applications have shed light on the relationship between the retention time of gel particles in the pore and filtration quality. Similar to an egg which slips through the neck of a bottle after a residence time of 200 seconds at a constant vacuum (differential pressure) level (Fig. 5), a gel particle will change its shape and slip through the pores following a lengthy residence time in the filter medium. The softer the gel particle, the more pronounced this effect becomes.

AKF and RKF series filters have a patented backwash feature which automatically removes gel particles from the metal fiber fabric. Backwashing can take place at regular 30 second intervals if necessary. This ensures that there is not enough time for the gel particles to be pressed through the pores. Filtration continues during the partial backwashing process.

More recent examples, including filtration of cellulose acetate for TFT film production, have shown that backwashing significantly increases film quality despite the fact that there was no increase in differential pressure which pointed to filter material loading. Suitable techniques for reprocessing the backwashed process medium are available for a wide range of filtered fluids.