Filter media packed beds

Filtration is the concentration of solids (or clarification of liquor) from slurry by fluid flow through a permeable medium. This normally takes the form of a membrane, filter leaf or packed bed, which restricts the particles, more than the fluid (Figure 4.4). 

A packed bed that consists of the same medium as that in Problem 3 is to be used to filter solids from an aqueous slurry. To determine the filter properties, you test a small section of the bed, which is 6 in. in diameter and 6 in. deep, in the lab. When the slurry is pumped through this test model at a constant flow rate of 30 gpm, the pressure drop across the bed rises to 2 psia in 10 min. How long will it take to filter 100,000 gal of water from the slurry in a full-sized bed... 

In cake filtration, the filter medium acts as a strainer and collects the solid particles on top of the initial layer. A filter cake is formed and the flow obeys the Carman-Kozeny equation for packed beds. 

The value of the permeability coefficient is frequently used to give an indication of the ease with which a fluid will flow through a bed of particles or a filter medium. Some values of B for various packings, taken from Eisenklam(2), are shown in Table 4.1, and it can be seen that B can vary over a wide range of values. It should be noted that these values of B apply only to the laminar flow regime. 

Figure 5.9.3 Schematic diagram of fixed-bed porous-glass-sphere culture system (1) packed bed of porous glass spheres (2) sampling port (3) medium fed in (4) air/oxygen sparge (5) off-gas filters (6) pH probe (7) dissolved oxygen probe (8) peristaltic pump (9) inoculation port (10) harvest.

On the other hand, in dead end filtration the retention is achieved by particle or gel layer buildup on the membrane and in the pores of the medium such as when a depth type filter is used. This condition is analogous to that encountered in packed-bed geometries. 

Fig. 3 (a) Drum bioreactor (7) substrate chamber, (2) loading port, (i) jacket, (4) paddles, (5) central axis, (6) motor, (7) gear box, (S) water inlet, (9) water outlet (70) air inlet, (77) air outlet, (72) product, (b) Packed bed bioreactor (7) water-bath where humidification occurs, (2) air inlet, (3,77) air filter, (4) sparger, (5) water input, (6) water filter, (7) heating coil, (S) basket with perforated bottom, (9) temperature probe for humidified air, (70) medium temperature probe (72) air output, (73) controller and recorder, (77) relative humidity regulator (7), temperature regulator, (A) airflow meter (W) water supply... 

Flow through packed beds of solids is usually analyzed by considering such characteristics as the porosity of the bed and the sphericity of the particles, and Section 7.5.4.1 shows that the analysis of a filter is helped by considering how the deposit of precipitated solids changes those characteristics. In the other filters, the solids deposit as a cake on the filter medium. The resistances of the filter cake and medium are then additive. When the resistivity, or the resistance per unit thickness, of the cake remains constant throughout operation, the specific resistance increases linearly with the amount of solid deposited. Analytical solutions for the filtration rate are then possible. In the constant-rate case, the pressure drop encountered can be expressed as a function of time (Section 7.5.4.2). 

The plant required for the above operations is relatively simple digestion is carried out in a stainless-steel vessel, steam jacketed, fitted with a stirrer, reflux condenser, conical base and bottom outlet. Special provision is made for passing a slow stream of hydrogen into the vessel via a pipe at the base. Filtration is on a Nutsche vacuum filter, a suitable filter medium being paper between two layers of terylene cloth. An additional clarification stage is also used to remove fine suspended particles of iron oxide. A bed of firmly packed paper pulp is satisfactory for this purpose. 

As an example of the application of the above analysis for flow through packed beds of particles, we will briefly consider cake filtration. Cake filtration is widely used in industry to separate solid particles from suspension in liquid. It involves the build up of a bed or cake of particles on a porous surface known as the filter medium, which commonly takes the form of a woven fabric. In cake filtration the pore size of the medium is less than the size of the particles to be filtered. It will be appreciated that this filtration process can be analysed in terms of the flow of fluid through a packed bed of particles, the depth of which is increasing with time. In practice the voidage of the cake may also change with time. However, we will first consider the case where the cake voidage is constant, i.e. an incompressible cake. 

The structure of the deposited cake resembles the structure of a packed bed. Based on the principles for flow through a packed bed with the consideration of resistances oflered by both the filter medium and the filter cake, an equation of cake filtration can be obtained (Akers and Ward, 1977 Cheremisinoff and Azbel, 1989 Tiller et al., 1987a) ... 

If such granules can be suitably contained (for example, in a packed bed or column), they will also act as a mechanical depth filter, trapping the suspended particles within the bed. Adsorbent filters can also be made by the addition of adsorbent sohds to a normal piece of filter medium, either as a layer sandwiched between two sheets of medium, or by embedding particles of adsorbent in the material of the filter medium, to make what is called a combination medium. 

The system involves operation of a filter with a deep bed of granular material as its filter medium, usually with the liquid flowing downwards under its own hydrostatic head. When full of dirt, the bed is normally cleaned by flow reversal, which expands the packed bed, releasing the trapped dirt particles and washing the dirt upwards and out of the vessel containing the bed. 

Granular bed filters Equipment that uses a bed of separate, closely packed solids as the separation medium. 

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