Cartridge Filters

Cartridge filters are used extensively throughout the chemical process industries in applications from laboratory-scale to commercial operations ranging to more than 5,000 gpm. Figure 1 shows a photograph of a series of units that serve as a pretreatment stage to an RO unit for water treatment. Quite often cartridge filters are used as a pretreatment stage, but occasionally they may serve as polishing filters and even stand-alone systems.

Table 1 lists typical filtering applications and operating ranges. The simplest and oldest types of designs include a series of thin metal disks that are 3-10 inches in diameter and set in a vertical stack with very narrow uniform spaces between them. The disks are supported on a vertical hollow shaft and fit into a closed cylindrical casing. Liquid is fed to the casing under pressure, from where it flows inward between the disks to openings in the central shaft and out through the top of the casing. Te suspended solids are captured between the disks and remain on the filter. As most of the solids are removed at the periphery of the disks, the unit is referred to as an edge filter. The accumulated solids are removed periodically from the cartridge.

Figure 1. Battery of cartridge filters used for water pretreatment in an RO process,

Table 1. Typical Operating Ranges of Cartridge Filters

Liquid

Filtration Range

Liquid

Filtration Range

Alum

60 mesh - 60 ¡j.m

Gasoline

1 - 3 ¡im

Brine

100-400mesh

Hydrocarbon wax

25 - 30 /im

Ethyl alcohol

5-10 ¿im

Isobutane

250 mesh

Ferric chloride

30-250 mesh

MEA

200 mesh - 10 ¿im

Herbicides and pesticides

100-700 mesh

Naptha

25 - 30 ¡im

Liquid

Filtration Range

Liquid

Filtration Range

Hydrochloric acid

100 mesh to 510 fim

Residual oil

25 - 50 fim

Mineral oil

1-10/xm

Sea water

5 - 10 fim

Nitric acid

40 mesh - 10 fim

Vacuum gas oil

25 - 75 fim

Phosphoric acid

100 mesh - 10 fim

Steam injection

5 - 10 fim

Sodium hydroxide

1-10 fim

Calcium carbonate

30 - 100 mesh

Sodium hypochlorite

1-10 fim

Dyes

60 - 400 mesh

Sodium sulfate

5-10 fim

Fresh water

30 - 200 mesh

Sulfuric acid

250 mesh - 3 fim

Latex

40 - 100 mesh

Synthetic oils

25 30 fim

River water

20 - 400 mesh

Other designs are simpler, experience lower pressure drop and have fewer maintenance problems. In an upflow cartridge filter, the unfiltered liquid enters the inlet (bottom) port. It flows upward, around and through the filter media, which is a stainless steel or fabric screen reinforced by a perforated, stainless steel backing. Filtered liquid discharges through the outlet (top port). Because of the outside-to-inside flow path, solids deposit onto the outside of the element so that screens are easy to clean. This external gasketing design prevents solids from bypassing the filter and contaminating the process downstream. There are no "o"-ring seals that can crack, channeling media that can fail or cartridges that can collapse or allow bypassing. As with any filter, careful media selection is critical. Media that are too coarse, for example, will not provide the needed protection. However, specifying finer media than necessary can add substantially to both equipment maintenance and operating costs. Factors to be considered in media selection include solids content, type of contaminant, particle size and shape, amount of contaminant to be removed, viscosity, corrosiveness, abrasiveness, adhesive qualities of the suspended solids, liquid temperature and the required flowrate. Typical filter media are wire mesh (typically 10 mesh to 700 mesh), fabric (30 mesh to 1 fim), slotted screens (10 mesh to 25 fim) and perforated stainless steel screens (10 mesh to 30 mesh). Table 2 provides some typical particle retention sizes for different filter media. Single filtration units can be piped directly into systems that require batch to semi-continuous services. Using quick-connector couplings, the media can be removed from the filter housing, inspected and cleaned in a matter of minutes. Filtering elements are also interchangeable. This means that while one unit is being cleaned, the other can be in service. Multiple filter units in a single housing are also quite common. These consist of two or more single filter units valved in parallel to a common header. The distinguishing feature of this type of configuration is the ability to sequentially backwash each unit in place while the others remain onstream. Hence, these systems serve as continuous operations. These units can be automated fully to eliminate manual backwashing. Backwashing can be controlled by changes in differential pressure between the inlet and outlet headers. One possible arrangement consists of a controller and solenoid valves that supply air signals to pneumatic valve actuators on each individual filter unit. As solids collect on the filter elements, flow resistance increases. This increases the pressure differential across the elements and, thus, between the inlet and outlet headers on the system. When the pressure drop reaches a preset level, an adjustable differential pressure switch relays information through a programmer to a set of solenoid valves, which, in turn, send an air signal to the pneumatic valve actuator. This rotates the necessary valve(s) to backwash the first filter element. When the first element is cleaned and back onstream, each successive filter element is backwashed in sequence until they are all cleaned. The programmer is then reset automatically until the rising differential pressure again initiates the backwashing cycle.

Table 2. Typica

Filter Retentions.

Mesh

Nominal Particle Retention

% Open

Size

(in.)

Otm)

Area

Wire Mesh

10

0.065

1650

56

20

0.035

890

46

30

0.023

585

41

40

0.015

380

36

60

0.009

230

27

80

0.007

180

32

100

0.0055

140

30

150

0.0046

115

37

200

0.0033

84

33

250

0.0024

60

36

Mesh Size

Nominal Particle Retention

% Open Area

(in.)

Gtm)

400

0.0018

45

36

700

0.0012

30

25

Perforated

10

0.063

1600

50

20

0.045

890

36

30

0.024

610

30

40

0.015

380

20

60

0.009

230

18

Slotted

80

0.007

180

25

100

0.006

150

13

120

0.005

125

11

150

0.004

100

9

200

0.003

75

7

325

0.002

50

5

Fabric

500

0.0016

40

Percentage of open area not applicable to fabric media.

-

0.0010-0.0012

25-30

-

0.0006-0.0008

15-20

-

0.0002-0.0004

5-10

-

0.000040.00012

1-3

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