Filtration is the separation of a suspension into the solids and the liquid (filtrate) by means of a porous filter medium. Both the solids and the liquid are moved to the filter medium by a driving pressure gradient.

The liquid can flow through the filter medium while the solids are retained. Depending on the type of technical implementation, a distinction is made between "cake filtration", "crossflow filtration" and "depth filtration".

Filter apparatuses most commonly used in industrial practice include continuous rotary filters such as disc filters, drum filters or pan filters. They are used for the cake-forming filtration of suspensions with solids concentrations in the range of 3–50 wt .- % and fulfill tasks such as

  • solids separation
  • solids washing
  • solids demoisturing

With BoVac and BoHiBar, BOKELA offers state-of-the-art filter series for these rotary filter types, both for continuous vacuum filtration and for continuous pressure filtration.

In crossflow filtration, in contrast to cake filtration, the formation of a filter cake is avoided. The suspension flows tangentially over the filter medium at high speed, thereby preventing particles from depositing to a significant extent as a cover layer. The liquid passes through the filter medium (e.g. a polymer membrane) and is withdrawn as permeate or filtrate. The solids remain in the concentrated but still pumpable suspension, called retentate.

With BoCross Dynamic and BoCross MicroScreen, BOKELA offers a unique crossflow filtration technology based on the principle of dynamic crossflow filtration. The BoCross filter series achieves outstanding results with microfine to nanoscale suspensions when it comes to

  • thickening
  • washing
  • clarification
  • classification.

Solids separation – the first step in filtration

The solids separation, meaning the separation of the suspension into solids and liquid, is the first process step of a filtration. The separation is carried out by a cake-forming filtration using a filter medium.

Driving force is a pressure difference Δp. While the liquid phase (mother liquor) flows through the filter medium and drains off as filtrate, the solid is retained by the filter medium and forms a filter cake. As few particles as possible should get into the filtrate. Depending on the process, either the mother liquor or the filter cake is the valuable product to be recovered.

The success of solids separation depends on product parameters, process parameters as well as apparatus parameters. Essential influencing factors are the properties of the particles, such as particle size, particle shape and particle distribution, the solids concentration of the suspension, the available driving pressure gradient Δp and the filter medium.

The finer the particles, the higher the flow resistance of the formed filter cake and the higher the required pressure differences Δp to separate the solids. With very fine particles, the available pressure difference Δp of vacuum filters of approx. 0.8 bar is often insufficient to overcome the flow resistance. For such suspensions, continuous pressure filters are used.

Whether a product can be processed on continuous rotary filters depends essentially on the formed filter cake being removable from the filter cloth. Filter cakes with special properties also require appropriate methods for cake removal.

CAKE WASHING

The washing of filter cakes is a process operation of great importance, as the requirements for the purity of products steadily increase. It is important to ensure a high washing efficiency with low wash liquor consumption and a sharp filtrate separation in order to minimize the costs of energy and washing liquid.

In many cases, the filter cake formed is washed with a washing liquid. The purpose of a filter cake wash is to remove unwanted substances from the filter cake in order to increase the purity of the solids or to recover desired substances. The recovered filtrate, the so-called wash filtrate can either be separated or removed together with the mother liquor. If the mother liquor is the desired product, in most cases it should not be diluted with the washing liquid.

One effective method of optimizing wash liquor consumption is countercurrent washing because the wash liquid is reused in multiple washing stages. The fresh washing liquid is added to the last washing stage, in which the filter cake is already prewashed. The wash filtrate from each stage is collected separately and returned to the previous wash step. The washing medium is thus moved on the filter cake against the direction of rotation of the filter. The sharper the separation of the wash filtrate in each wash step, the more efficient the use of the fresh wash liquid in the last step and the less wash liquid is needed.

 

The main requirements for effective countercurrent cake washing on rotary filters are:

  • homogeneous filter cake, to prevent washing liquid from flowing through few large pores without any washing effect (fingering)
  • an optimal washing water distribution over the filter cake
  • rapid and complete filtrate drainage from filter cells and filtrate pipes to prevent carryover of filtrate to the next process zone
  • a reasonable number of filter cells, which allow for a precise distribution of the process zones.

A particularly intensive washing of filter cakes is achieved with BoHiBar steam pressure filtration. In this hybrid process, the filter cake is steamed in a special steam hood. It forms a "condensate front", which flows through the filter cake and thereby ensures an intensive displacement of the mother liquor.

CAKE DELIQUORING

Further processing of the filter cake formed during filtration requires low residual moisture in most applications. The solids should be as dry as possible. Liquid that remains in the filter cake often has to be removed by thermal drying. The energy consumption is up to ten times higher than with a mechanical deliquoring of the filter cake on the filter apparatus.

In the last process step, after the filter cake wash or already after the solids separation, the filter cake is deliquored by air flowing through it. It is often the goal to produce a filter cake that is as dry as possible. This can have two reasons: the further processing of the solid requires the lowest possible residual moisture content, or the liquid phase is the valuable product and should be recovered as completely as possible.

Too high residual moisture in the filter cake can cause high post-treatment or energy costs. Frequently, liquid remaining in the filter cake has to be removed in a downstream dryer with an energy consumption that is almost ten times higher than would be possible mechanically with a modern filter of intelligent design and correct operation.

After deliquoring, the filter cake is removed from the filter fabric via a corresponding cake discharge device and fed to the subsequent process as well as the resulting filtrates.

As with solids separation and cake washing, the success of cake deliquoring depends on product parameters, process parameters as well as apparatus parameters.

Homogeneous filter cakes are a prerequisite for the fact that the drying air flows evenly through the filter cake, penetrates as many pores as possible and does not flow off ineffectively through a few large pores (fingering).

The finer the particles, the higher the flow resistance of the formed filter cake and the higher the required pressure differences Δp for cake demoisturing. Thus, pressure filters usually achieve lower residual moisture than vacuum filters.

Particularly low residual moisture is achieved with BoHiBar steam pressure filtration. In this hybrid process, the filter cake is steamed in a special steam hood. Synergy is released through the combination of mechanical and thermal energy. The deliquoring process is accelerated and intensified.

CLASSIFICATION

By classification, different fractions having different particle properties are produced from a mixture of solids, or coarse/fine particles are removed which are undesirable in subsequent processes.

Classification or screening means the separation of a particle collective into different fractions, which usually differ in their particle size or particle type. The aim of the classification is

  • to produce several – but at least two – subsets of the original solid mixture
  • to separate coarse or fine particles (oversize or undersize grain) that interfere with subsequent processes.

For many products, sales opportunities or further processing within a process depend on maintaining a certain purity or a narrow particle size distribution. Thus, the reliable separation of interfering particles is of immense importance, especially in the production of high-quality, expensive products, such as in polymer chemistry, the food industry, biotechnology, the pigment industry, the ceramic industry, materials technology, etc.

The separation of interfering particles from suspensions with critical properties such as very high particle concentrations, high viscosity, thixotropic behavior or soft particles etc. represents a particular challenge in separation technology.

With the BoCross MicroScreen filter, high viscous and highly concentrated dispersions can be continuously sieved in a closed apparatus with high throughput and sharp cut, while the retentate content and thus, the product losses are extremely low.

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