Crossflow Filtration with a Sieve Filter

BoCross MicroScreen filter – screening under difficult process conditions.

The BoCross MicroScreen filter offers unique continuous process solutions with regard to classifying suspensions or to fully recovering or separating solids from suspensions with challenging product or process characteristics.

Like the BoCross Dynamic filter, the BoCross MicroScreen filter works on the principle of dynamic crossflow filtration. However, backwashable asymmetric metal media are used as filter media. This way, innovative separation processes are realized.

The dynamic sieve filtration with the BoCross MicroScreen filter is a method for the classification of suspensions such as the separation of disturbing coarse grain from very particulate suspensions or the separation of interfering fine grain fractions. For the first time, highly viscous and highly concentrated suspensions can be continuously sieved in a closed apparatus with high throughput and with a sharp cut point. The product losses are very low.

Dynamic precoat filtration is a combination of cake forming filtration and crossflow filtration. With this method, solids can be completely recovered from a suspension even under difficult process conditions where membrane filters or centrifuges fail. The filtrate is absolutely particle-free. The dynamic precoat filtration allows, for example, a circulation of catalysts with a very short residence time outside the reactor.

The BoCross MicroScreen filter is available with filter sizes from 0.1 m² to 12 m² and can be used for an operating pressure of up to 6 bar (a) and an operating temperature of up to 100 °C. In special cases, higher pressures and temperatures can be realized.

Applications of the BoCross MicroScreen filter:

Features and benefits:

  • separation of interfering coarse or fine grain fractions by continuous classification of suspensions (dynamic sieve filtration)
  • complete recovery of solids from suspensions and particle-free filtrates through dynamic precoat filtration especially where membrane filtration is not recommended
  • high solids concentration and high viscosity allowed in the feed
  • continuous operation
  • hermetic process

Dynamic sieve filtration with the BoCross MicroScreen filter is a method for classifying suspensions. The fine fraction and the mother liquor pass through the sieve medium, while the much smaller coarse fraction is retained by the sieve and discharged as a highly concentrated concentrate. This way, products of high purity or a narrow particle size distribution can be produced. To avoid depth filtration, the screen is cleaned at regular intervals during operation by short, intermittent backwashing.

The strengths and advantages:

  • continuous separation of low concentrated coarse grain (ppm range)
  • cut size down to 5 μm
  • minimal product loss through 20 to 500-fold coarse grain accumulation in the retentate
  • high solids concentration in the feed is allowed
  • screening even with high viscosity and thixotropic flow behavior
  • screening without air contact

Dynamic precoat filtration allows solids to be completely recovered from a suspension. This process combines dynamic crossflow filtration with classic precoat filtration. For this purpose, the speed of the rotors is adjusted so that under controlled conditions, a particle layer of typically 1–2 mm forms on the filter medium. The metallic filter medium only serves as a carrier for the precoat layer, which represents the actual separation medium for this process. filter aids from other material are not needed because the precoat layer is made of the particles of the product suspension. At certain time intervals, the precoat layer is removed by backwashing.

The strengths and advantages:

  • complete separation or recovery of the solid (e.g. catalyst)
  • constant filtrate flow
  • particle-free filtrate (after a short start-up time)
  • low residence time of the product in the filter
  • can be used in difficult separation tasks where membrane filters or centrifuges fail, such as
    • hot suspensions
    • suspensions containing solvents and / or fine particles.

The BoCross MicroScreen filter is very gentle on the product. The desired process result is achieved not only after several circulations with circulating streams but in only one run with short residence times. The variable rotor speed ensures that no particle destruction takes place. A heating/ cooling system which is integrated in the modules allows a tempering of the product by cooling or heating during filtration.

The BoCross MicroScreen filter reacts flexibly on changes in the product characteristics – for example, in terms of viscosity or solids concentration in the feed or as a result of product changes – and thus, ensures reliable operation and constant operating results.

For both processes, asymmetric metallic filter media are used which allow backwashing. For dynamic sieve filtration, separation media are used with a cut size between 20 and 100 microns. They have excellent selectivity and outstanding service life.

BoCross MicroScreen filters work fully automatically and can be easily integrated into process automation. Startup and shutdown procedures as well as cleaning procedures are automated.

Filter Type [–] XS01 M2 XL10 XL12
Filter Area [m²] 0.13 1.8 10 12
Filter Diameter [mm] 145 375 850 850
No. of modules [–] 5 10 10 12
Drive [kW] 3 < 15 > 45 > 45


Plastic Precursors

Filtration Apparatuses for High Product Quality and Consistently Low Costs.

Filtration is often an important step in the production of plastics. BOKELA products such as BoHiBar Drum, BoVac Drum or BoCross Micro Screen provide reliability, long service life and excellent process results. Minimal energy consumption and reduced drying costs ensure an improved economy.

Pigments, Fillers & Dyes

BOKELA filters for Sensitive Solids and Smallest Particle Sizes.

Pigments, fillers and dyes are used very differently in the various fields of application. A feature all of them share is the very small size of particles, usually smaller than one micrometer. The technologies of the BoCross Dynamic, Boris MicroScreen, BoVac Drum and BoHiBar Drum series ensure a reliable and product-friendly separation.

Catalyst Recovery. BoCross MicroScreen

A Solution with Many Advantages.

Catalysis is an important process step in almost 90 % of all production processes in the chemical sector. The separation and recovery of solid catalysts from fluid phases is a real challenge – for the separation apparatus and the filter medium. BoCross MicroScreen meets these high demands.

Crossflow Filtration. For Difficult Suspensions

Macro, micro, or nano.

BoCross Dynamic and BoCross MicroScreen are dynamic crossflow filters for micro, ultra, dia or sieve filtration. With the BoCross filter series, suspensions with high concentrations or viscosities can be thickened, washed, cleared or classified.

Recovery of Catalysts by Dynamic Precoat Filtration

In a BDO production plant (butanediol process) 24 BoCross MicroScreen filter units of 10 m² each are applied for recovery of catalyst by dynamic precoat filtration.

Dynamic Sieve Filtration of a High Viscous Polymeric Suspension

From a high valuable polymer dispersion coarse particles (x > 20 µm) have to be separated completely. The difference in density of the particles compared to the fluid is negligible.

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