Special device used at yeast filtration on precoat drum filters to remove surplus suspension from the filter cake surface when emerging out of the suspension. The cake equalizer limits cake formation and adjusts the formed filter cake to an optimum thickness for subsequent cake washing and cake demoisturing.
is a surface filtration process representing the third basic filtration type, beside deep bed filtration and crossflow filtration, respectively. Its objective is to retain the solids in a suspension as a bulk on the upper side of a filter medium. Both liquid or solids can principally be the desired product. Cake filtration requires a certain critical solids concentration in the feed to build particle bridges across the pores of the filter medium, which then act as the active filter medium. The driving potential for the cake filtration can be a gas differential pressure, a mechanical or hydraulic pressing power, or a centrifugal pressure. Hence the spectrum of cake filtration equipment is wide. The cake filtration allows washing of the bulk and its mechanical demoisturing after the cake formation process. The formation of filter cakes can be described by the cake formation equation.
Cake Formation Angle
Angular sector available for cake formation in a drum filter, disc filter or pan filter. The cake formation angle α1 is connected by the number of revolutions n with the cake formation time t1 as follows:
Cake Formation Equation
For constant feed ﬂow rate it has the form below:
In most practical applications, the filter cake can be assumed to be incompressible and the filter cloth resistance Rm is much smaller than the cake resistance rC. Under these conditions, equation 1 can be simplified to following equation:
According to this simplified equation the cake height hC is a linear function of the square root of the cake formation time t1 and forms a straight line when plotted in a diagram of hC over the square root of t1. The validity of this simplification can easily be proven by carrying out 3- 4 tests at constant pressure and different filtration times.
hc [m] = cake thickness, rc [1/m²] = speciﬁc cake resistance, Rm [1/m] = ﬁlter cloth resistance, κ = ‚ Kappa-Factor, Δp [N/m²]= pressure difference, t1 [s] = cake formation time, A [m²] = ﬁlter area, ηL [Ns/m²] = dynamic viscosity, VL [m³/s] = ﬁltrate volume ﬂow.
Speciﬁc permeation resistance rc of the filter cake. Cake resistance rc contains globally all the information about the inner pore geometry and structure of a ﬁlter cake, which cannot be calculated theoretically but must be measured directly as a material function. It is independent of the cake thickness and can be directly determined via Darcy’s law by flowing a particle-free liquid through two filter cakes of different heights at a predetermined pressure and measuring the flow rate. From the resulting two equations the filter cloth resistance can be eliminated. Normally, the cake resistance rc is determined directly by the t/V=f(V)-method.
Method for the removal of mother liquor and dissolved substances from a filter cake with a liquid, mostly miscible with the mother liquor. One differentiates between displacement washing and dilution washing; the latter involves re-suspending of the filter cake in the wash liquor. Quality criteria for displacement washing are a high wash degree, and low consumption of wash liquid which can be expressed by the wash ratio.
is a thermal-mechanical surface treatment process for smoothening of woven surfaces. A weave made of thermally workable material is treated with pressure in a calender roller to give a smooth surface to a filter medium, which for example facilitates cake discharge. It should be noted that the resulting pore size of the filter surface is decreased by calendering.
Discontinuously working filter equipment designed as either cake filter or deep bed filter. Often, cylindrically shaped filter elements are manifolded together in large number in a pressure tank to house an economically reasonable filter area in a given vessel volume. Candles covered with a filter cloth are applied in conventional cake filtration or in precoat filtration. They can discharge a dry cake after gas pressure demoisturing or a highly thickened suspension after dropping it in the surrounding heel (e.g. Fundabac filter). If candle filters are employed as deep bed filters they serve for the purification of liquids polluted with small amounts of contaminants. So-called rolled candles and elements made from sintered materials are also employed in candle filters.
Candle Press Filter
Special type of membrane filter press, at which the press membrane is arranged in a circular, cylindrical manner around a filter candle. This special design variant allows pressing forces of considerably exceeding 100 bar; it is used in extremely difficult to filter products, e.g. in the field of biotechnology.
The behavior of liquids caused by interfacial tension.
Capillary Belt Filter
Filter machine in which the liquid flows out of a bulk material through the openings of a vibrating screen where it is received by an absorbing belt underneath, circulating against the solids transport direction. The absorbent medium is then squeezed between two press rollers and available again for a liquid intake upon return.
Capillary Entry Pressure
is the pressure difference across a curved gas-liquid phase boundary, which is compensated by the interfacial tension. The capillary pressure in the pores of a ﬁlter cake holds the liquid in it. A capillary pressure can have positive or negative values. Accordingly, a capillary pressure is positive if a lower pressure exists in the phase with the larger. Therefore, the capillary pressure of a liquid droplet in a gaseous environment is negative, whereas the capillary pressure of a gas bubble in a liquid would be positive.
Capillary Pressure Curve
is a function that describes the capillary pressure distribution within a bulk due to the underlying pore size distribution. The capillary pressure is thereby ploted against the saturation degree. This function deﬁnes what minimum saturation degree can be reached at equilibrium for each pressure difference that is constantly applied on the bulk from the outside. Special values of the capillary pressure curve are the capillary entry pressure and the remanent saturation. The capillary pressure curve can be determined for an applied gas difference pressure or for a centrifugal ﬁeld; both measurements results should be theoretically the same. The capillary pressure curve in homogeneous bulks does not depend on the physical height of the bulk.
Capillary Pressure Distribution
Capillary Suction Draught
Carman & Kozeny-Equation
is used to quantify the permeation velocity v in bulk solids analogous to Darcy’s law, however, it explicitly describes the inﬂuences, respectively, of the po-rosity ε, the volume speciﬁc solids surface SV, the ﬂuid viscosity η, the driving pres-sure difference Δp, and the ﬁlter layer thickness hC:
The value of k(ε) is constant at around 5 in the porosity region of ca. 0.3 ≤ ε < 0.65 . k(ε) has to be determined precisely by a calibration measurement. The equation em-phasizes the strong inﬂuence porosity has on the permeability of bulk materials. A similarly structured relation is given by the Gupte-Equation.
A positively charged ion in a solution (comp. anion)
Cell Belt Filter
Cell Drum Filter
Cell Less Drum Filter
with a non-partitioned interior that is completely exposed to a vacuum. For the cake removal a so-called control shoe with sliding seals is pushed in the removal zone against the drum’s inner wall for cake removal by compressed air backpulse. The filtrate in turn is withdrawn from the base of the drum’s internal space by a ﬁltrate suction pipe.
Cleared liquid evolving from a centrifuge.
Special design of a disc stack separator used for the separation of two immiscible liquids with different densities. The heavy phase is discharged with a peeling disc by adding extraction agents into the mixing chamber. A complete and spontaneous mixing of the extraction agent is achieved through the peeling disc.
filter centrifuge with a perforated drum that is covered with a filter medium. Centrifugal filters can demoisture cake solids to a great extent by removal of the fluids that are held by capillary forces and can produce a free-flowing solid product.
A particle in circular motion strives to leave the circle in the direction of the tangent to its path according to the law of inertia. A force has to be exercised on it constantly, pointing to the center of the circle, called the centripetal force. According to the counter effect principle the centrifugal force corresponds to this as an oppositely directed, equal-sized force. It represents the inertial resistance with which the moving particle opposes the constant change of the direction of its path.
A rotating mixing chamber fed centrally through concentric pipes with liquid com-ponents and equipped with a stationary peeling disc for the discharge of the mixed liquid.
solid-liquid or liquid-liquid separation process in a centrifuge.
Rotating separation apparatus employing centrifugal forces as driving potential. Mass forces are produced that act both on the particles to be separated as well as on the liquid contained in the centrifuge. Centrifuges are used for sedimentation and filtration. The range of C-values in commercial centrifuges starts in the low 100’s and can go up to several 10,000.
Ceramic Disc Filter
A continuously operating vacuum filter based on a disc filter. In place of conven-tional filter cells the filter elements are ceramic plates filtering on both sides. The microporous filter plates have approx. 1µm pore size, are each several mm thick, and therefore impose a high ﬂow resistance that controls the filtration process. The minute pore width is advantageous for producing filtrates free of particles and without gas ﬂow in the vacuum region. The capillary pressure of the hydrophilic ceramic media is larger than the applied pressure difference, so that the pores remain always wetted.
Chain Assisted Cake Removal
Special type of filter cake removal in drum filters where parallel running chains or strings (string assisted cake removal) are guided around the filter drum and are led away from the drum in the cake removal sector. Following a sharp deflection by a roller system the chains are routed back to the drum. The filter cake consisting of mostly fibrous solid particles builds up around the chains. Therefore the cake is picked up in the removal sector of the drum by the chains like a fleece and is thrown off at the sharp chain deflection.
Chamber Filter Press
The most common design variant of the filter press filter where two neighboring filter plates with a one-sided cavity face each other to form a filter chamber. The filter medium is stretched over each plate and is pressed into the chamber during the process. The discharge of the cake from chamber filter presses can be executed far more beneficially and simpler than from frame filter presses. However, since the cake discharges due to its weight by dropping out of the chambers, a certain cake thickness of several centimeters is required, which tends to lengthen the cycle. In addition to this disadvantage, a further problem can be that residual volume filtration will not be possible, if the suspension feed stops when the chambers filling has not been completed. In this regard the membrane filter presses are clearly at an advantage.
Phenomenon observed during sedimentation of particles in the region of swarm sedimentation. At certain concentrations (intermediate suspensions) hydrodynamic instabilities can occur that form particle-free liquid channels parallel to the sedimentation direction. Similar effects are known for the flow through fluidized beds.
Food-suitable flocculation agent produced from shells of crustaceans.
is defined as the complete separation of all particles from a dilute suspension (e.g. beverage), most frequently done by deep bed filtration.
Special design of a disc stack separator for separating solid particles and liquids. Herein the disc package does not contain any rising channels, as in the disc stack separators, which are conceived for the separation of two non-miscible liquids with different densities and potentially solids, too. Clarifying separators are designed with respect to the feed solids content as nozzle-type separators or as self-cleaning separators. They are able to reach C-values of more than 15,000 and have equivalent clarifying areas of up to 100,000sq.m. Clarifying Separators are employed for the separation of extremely small particles down to the sub-μm region.
Cleaning In Place
Method to clean an apparatus without dismanteling by internal spray nozzels, which are able to reach every location in the process room to be cleaned.
commonly refers to the overflow of gravity thickeners with as few particles as possible.
Clear Liquid Zone
Nearly particle-free zone developing in the upper section of a sedimentation tank (gravity thickener), from where the separated liquid is taken off with an overflow.
Phenomenon in the cake filtration under the influence of gravitational or centrifugal forces, observed as a partition of the suspension with respect to its particle size. The larger solid particles settle quickly on the filter medium according to Stokes’ law, whereas the smaller particles are deposited later on the cake surface forming a so-called clogging layer. Often, this clogging layer is highly impermeable and it will then increase the capillary entry pressure and thus the residual moisture of the resulting filter cake.
Critical Micelle Concentration Micelles
Co-Current Flow Decanter
Special ﬂow distribution of the centrate in a decanter centrifuge. The feed suspension enters at the cylindrical end of the centrifuge, so that solids and centrate can move together towards the conical end. From there the centrate is re-turned by channels attached along the structure of the ‚ transport screw for di-scharge from the cylindrical end. The purpose for this design is to disturb the sett-ling process of the solids as little as possible. It affords readily a high separation degree, i.e. a very clear centrate, with solids that tend to get re-suspended easily by eddies.
is the simplest way of carrying out displacement washing or dilution washing where the wash liquid is brought only once into contact with the particle system to be washed. This method consumes more washing liquid than counter-current washing. The latter, however, is not feasible with all types of separation equipment. In centrifuges for example only the co-current washing method can be employed due to a lack of possibilities for segregating the centrate.
Phenomenon of emulsion instability. If two droplets of an emulsion get into contact, they can aggregate to one bigger drop. This is a demixing effect. One can stabilize emulsions against coalescence by surfactants (emulsifier).
Coarse Capillary Liquid
is defined as the major portion of liquid in a saturated bulk that is held between the particles in hydraulically inter-connected voids. The coarse capillary liquid is readily accessible to mechanical demoisturing by gas differential or centrifugal pressure. Beside the coarse capillary system exists also a fine capillary system.
Coarse Capillary System
Equipment employed in sewage treatment for the screening of coarse particles in the cm-size region, such as paper, wood, plant refuse, plastics. The screen is periodically cleaned off the accumulated debris with an automatic, comb-like device.
So-called colloidal disperse systems are aggregates of molecules, comprised 103 up to 109 molecules; they would have a diameter of 10-7 to 10-4 cm if they assumed a spherical shape. Colloids assume an intermediate position between the molecular disperse and the coarsely disperse systems. They are difﬁcult to separate in stable dispersions and cause turbidity in a separated clear liquid.
In-series arranged solid-liquid separation machines for dividing the separation pro-cess into sections of different liquid content. A typical combination may consist of a pre-thickener (e.g. static circular thickener), followed by a mechanical demois-turing apparatus (e.g. vacuum drum filter), and last, a thermal drying step (e.g. spin ﬂash-dryer). Through proper serial combination of separation equipment a desired separation can be technically realized and at the same time economically optimized.
consisting of at least two different materials which are solidly attached to each other. Composite membranes are employed especially in ultrafiltration which demands extremely small membrane pores for the material retention. They consist of a ﬁne porous, very thin cover layer with high ﬂow resistance and a large-pored, mechanically stable support layer underneath. Weaves and fleeces are often used as a support layer.
Compressed Air Blow Back
The energy source for detaching a filter cake from a filter cloth at the solids discharge of drum, disc, candle and leaf filters. After demoisturing a sudden increase of pressure is generated on the filtrate side behind the filter medium, in order to break the cake up or cast it away. A critical point with pressurized air blowback is the potential for filtrate residues remaining in the cloth or in the filter cell, which can consequently cause re-moisturizing of the filter cake.
The property of a bulk to decrease its void space due to a load acting from the outside. The dimensionless compressibility degree U is used to quantify compressibility. U relates the difference between the original layer thickness L1 and a present layer thickness L to the maximum compression (L1 -L∞), which in turn is the difference between original layer thickness L1 and the layer thickness reachable at equilibrium L∞:
Process employed in solid-liquid separation technology for the demoisturing of bulk materials, which do not have a rigid pore matrix. The liquid is squeezed to the outside by decreasing the pore volume, so that the dry substance content increases. Compression is technically preferably achieved with the aid of press filters. During sedimentation, the developing bulk is furthermore subject to compression by the acting mass forces. Compression does not only occur with elastic but also with rigid particles. There are four different modes of compression: first, the particles approach each other; secondly, they glide past each other; third, further compression can be achieved to a certain extent by rearranging the particles, and fourth, a particle fracture occurs.
Term used in the ﬁeld of sedimentation. The compression layer in a thickener deﬁnes the region where the settled particles at the bottom of the tank ap-proach each other to such an extent that they are capable of transferring mechani-cal forces among each other. The sediment in this region is compressed due to its weight. The higher the layer and the longer the time spent the stronger the com-pression.
Energy W, which has to be applied for the compression of a gas with the volume Vg1 (air) by the pressure pg1, in order to generate and maintain a pressure difference (Δp = pg2 – pg1), necessary for vacuum or overpressure ﬁltration. It is calculated for an isentropic change of state with the isentropic coefﬁcient κ:
Term out of the ﬁeld of crossﬂow filtration. At the permeation of the filter a con-vective transport of the materials to be retained develops in the direction of the membrane due to the pressure difference across the membrane. In the course of this developing concentration gradient, a diffusive back transport into the core ﬂow results. In the stationary case an equilibrium between these two processes is reached.
Phenomenon at the Steam Pressure Filtration on BoHiBar Filters which provides for excellent ﬁlter cake demoisturing and filter cake washing. A condensate front is formed when a “cold” and saturated filter cake enters the hot atmosphere of overheated or saturated steam in a steam cabin. Then, the steam conden-ses on the surface of the cake and the condensation enthalpy heats the cake surface up to the condensate temperature. While the pressurized steam forces the mother liquor and the condensate through the cake, cold regions of the ﬁlter cake come in contact with steam and further condensate is formed. This leads to a sharply deﬁned and evenly developed condensate front which moves through the cake as a homogenous condensate layer preventing a fingering. Thereby, the mother liquor is displaced and the cake is completely heated up to the temperature of the condensate. This mechanism combines heat and mass transfer between filter cake and steam.
The changing of the properties of a suspension, a sludge, or a filter cake. Suspension conditioning is defined as a pretreatment for the improvement of the separation characteristics. This can be for example a flocculation by addition of a polymer. The sludge conditioning can also include an agglomeration process for the improvement of additional demoisturing steps. Slurries or filter cakes can be conditioned with additives, such as lime, to increase their dry substance content as required for landfilling, for example.
The contact angle δ characterizes the wetting of solids by a liquid. One talks about wetted conditions, if the contact angle, measured always in the fluid phase with a higher density, is < 90°; is it > 90° one talks about non-wetted conditions.
Contact Point Number
Count of contact points a particle has with neighboring particles in a bulk material. In regular packages, exactly defined in geometric terms, the contact number is fixed. Thus a particle in densest possible sphere package possesses 12 contact points to its neighboring particles. The contact point number influences the bulk properties, especially in the region where the liquid is bound essentially only in the form of liquid bridges. It influences the tensile strength and the saturation degree of the moist bulk.
In a mixture out of solids and liquid for example one of both substances represents depending on the concentration the continuous and the other the discontinuous or disperse phase. The continuous phase is connected together and encloses the discontinuous phase. In a suspension the liquid presents the continuous phase, whereas the dispersed solids forms the discontinuous phase. By compari-son at a filter cake the connected solids structure forms the continuous phase and the liquid is distributed as a discontinuous phase in the ‚ pores of the bulk.
Continuous Separation Apparatus
Separation apparatus which is equipped with a constant feed for the suspension and steady discharges of separated liquid and moist solids. The individual steps of the separation process therefore are synchronized in terms of transport velocity and the geometry of the apparatus and are not independently adjustable. Continuously working machines are typically employed for large mass ﬂows and continuous production processes. Decanter centrifuges or drum filters are examples for continuously working separation apparatus.
A disc made out of special grade cast iron or HDPE, employed at the control head of rotary filters, into which the control zones are cut as slots. The control disc is stationary while the filtrate pipes move from control zone to control zone (e.g. cake formation zone, demoisturing zone, compressed air blowback zone, cloth cleaning zone).
Interface between the stationary and the rotating part during the draining off of filtrate at rotary filters. A control head is divided into different control zones, in order to adjust independently and separately from each other the pressure difference in the cake formation and demoisturing region, as well as the compressed air blowback for the cake discharge, and possibly a bubbling zone. Respective to their design, one can differentiate between axial and radial control heads. In the axial type the division into zones is performed by the so-called control disc into which the slot-shaped control zones are cut. The control head sits flat with seals against the rotating counter part of the integrated ‚ filtrate pipes. In a radial control head, the rotating part moves concentrically in a stationary ring with a channel, which can be divided by so-called separation plugs into separation zones. The sealing is with gland packages. In industrial practice the axial control head is preferred due to its easier sealing and lower design complexity.
Discharge device for solids (discharge chute), usually designed in the form of a pipe, through which the separated and demoistured solids leaves the process chamber of the separating apparatus under the inﬂuence of the gravitational force. The requirement for the functioning of a conveyer chute is a sufﬁcient pourability of the product. If this tends to stick, then one selects a discharge screw instead of a conveyer chute.
Discontinuously working laboratory centrifuge with integrated cooling of the beakers. Especially with high speed centrifuges in the region of several thousand g the rotor is heated by the friction of surrounding air. If a constant temperature is desired, for example due to probable product destruction other wise, the rotor has to be cooled.
is acting during a relative movement of a particle against a rotating reference system in the centrifugal field. It is an inertia force usually possessing a negligibly small value.
Coulomb‘s Friction Law
Process for the removal of unwanted solutes from suspensions or filter cakes, especially for saving wash liquid. The counter-current washing can be applied in displacement washing as well as in dilution washing. The fresh washing liquid is added at the end of the separation process in order to clean the already almost completely cleaned product. The washing liquid by this now already enriched with some mother liquor is then utilized for the puriﬁcation of the even stronger contaminated product located further in the direction of the process beginning. Finally, the highly enriched wash liquid is discharged out of the process.
Counter Flow Decanter
Conventional and simple design of a decanter centrifuge. The suspension is fed approximately in the middle between the cylindrical and the conical end of the centrifuge. Solids and centrate stream then in a counter flow. While the transport screw conveys the solids into the direction of the conical end, the centrate flows in the opposite direction to the cylindrical end, where it leaves the centrifuge for example over a weir disc.
Layer of ions, relative strongly bound to a charged particle surface, out of the sur-rounding liquid with a particle surface of opposing charge. With increasing distance from the particle surface the concentration of the counter ions, compensating the surface charge, decreases. According to the DLVO-Theory the compensation of the particle charge by the counter ions determine to what extent particles are capa-ble of agglomerating due to their attracting Van-der-Waals forces.
Discontinuously, in a pressure tank under overpressure working cake filter. The ﬁlter elements are similar to filter candles but however display a ﬂattened shape. They thus represent a connecting link between candle filters and leaf filters. Several ﬁlter elements are comprised in a register. In a pressure tank several regis-ter are arranged parallel. Cricket filters are suited for dry- and wet discharge of a product and can be employed as a precoat filter. The cricket filter is a typical representative of a fine filter for suspensions difﬁcult to filtrate.
Soaking of weaves in boiling water, applied a pre-treatment, e.g. with cotton fabrics to render them resistant to shrinking.
comprises all ﬁltration techniques, where the suspension to be separated is lead under a pressure in parallel ﬂow to a filter medium, in order to prevent the deposition of solids on the filter medium as much as possible. Due to a pressure difference, the permeate penetrates the filter medium. The concentrate leaves the process chamber highly concentrated but still free ﬂowing. At dynamic crossﬂow filters the shear ﬂow can additionally be increased by stirring elements (BoCross-Filter of the BOKELA company). Microporous membranes are usually employed as ﬁlter media. The ﬁeld of application for crossﬂow filters is vast and ranges from upgrading of sewage to product isolation at biotechnological productions. The production of high-purity liquids (sterile filtration) from suspensions with often extremely low particle concentrations in the µm- and sub-µm region and poor ﬁltration behavior is characteristic. Both precoat filtration and disc stack separators are in competition with crossﬂow filtration.
Special packaging of a membrane filter medium in a micro- or ultra filtration unit. Here two membrane discs are welded together at the outer edge. Between both membrane discs a spacer is situated through which the permeate is discharged by means of a permeate drain off pipe, connected tightly with the membrane cushion. Designs with circular or rectangular membrane cushions are known. In the case of a circular membrane cushion the permeate flows to a centered discharge pipe. Several membrane cushions are combined in a pipe-shaped casing to a membrane module.
states by how many times the centrifugal acceleration in a centrifuge surpasses the gravitational acceleration g:
r = centrifuge radius, ω = angular velocity, n = revolution number of the centrifugal drum, d = centrifuge diameter. C-values at industrial centrifuges can reach values of several 100 up to several 1,000.