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Wet Scrubber Packing: Types, Characteristics, Materials & Column Arrangement

Packed columns are among the most common types of mass transfer equipment. Wet scrubber packing refers to a layer of contact elements of a specific shape, which ensures the presence of channels within the packing through which the phases involved in mass transfer move. One of the phases, when moving along the packing, spreads over the surface of the contact elements, thereby increasing the interfacial area. In packed columns, the liquid film flows along each element, then breaks up and forms again on the next element. Thus, the wet packed scrubber operates under a film flow regime.

Key Characteristics

There are two main characteristics of wet scrubber packing media: porosity and specific surface area of the elements.

The porosity of the packing (ε) is the fraction of free volume in the total volume of the packing. The higher the porosity, the easier it is for the phases to move through the packing, i.e., the lower its hydraulic resistance, which reduces the energy required to move the phases.

The specific surface area (a) is the total surface area of the packing elements divided by the volume occupied by the packing. The higher the specific surface area, the greater the contact area between the phases and, consequently, the more efficient the wet scrubber filler becomes.

Packed Bed Wet Scrubber Design and Types of Packing

It should be noted that the surface area is not identical to the interfacial contact area between phases. On one hand, not the entire surface of the packing transforms into interfacial area due to incomplete wettability of the element surfaces. Here, wettability refers to the distribution of one of the phases over the packing surface in the form of a film. On the other hand, the interfacial area can be formed not only by the liquid film on the packing surface but also by droplets and jets detached from the surface.

Porosity and specific surface area of the packing are directly related to the shape of the elements. In general, the more complex the shape, the higher these characteristics. However, it should be considered that the complexity of the element shape should not reduce its wettability or significantly increase manufacturing costs.

Types

Depending on the arrangement of the elements in the equipment, packing can be classified as structured (regular) or random (irregular).

Structured packing consists of elements that are properly arranged, which ensures better wettability. Examples include grid, ring, and block types.

A packing layer is considered random when the elements are arranged chaotically, as occurs when they are poured into the equipment. All else being equal, this arrangement has a lower specific surface area than structured packing, but it is easier to load. Irregular packing can be in the form of dumped pieces, rings, saddles, propellers, or twisted (wire) elements.

Rings

The elements of ring packing are cylindrical thin-walled rings, the outer diameter of which is usually equal to the height of the ring. Ring diameters range from 10 to 150 mm (elements of smaller diameter are rarely used in industrial practice). They are most commonly made of ceramics or porcelain, and in some cases from carbon-graphite materials. Thin-walled metal rings made of steel or other metals are also used.

Fig. 1

Raschig rings (Fig. 1) are simple rings without any additional features. They are the cheapest and easiest to manufacture, have proven effective in practice, and are the most common type of packing. Raschig rings can be arranged both in a structured layer (for diameters above 50 mm) and in a random layer (for diameters from 10 to 50 mm). Methods of arranging Raschig rings are shown in Fig. 2.

Fig. 2

Pall rings (Fig. 3) are primarily intended for random filling and have lower hydraulic resistance and slightly higher efficiency compared to Raschig rings. They are rings with cutouts and internal protrusions, which improve the wettability of the randomly packed layer.

Fig. 3

Other types of rings are also used. To increase surface area, rings with partitions (Lessing rings) (Fig. 4a), rings with cross-shaped partitions (Fig. 4b), and spiral rings with one, two, or three internal spirals (Fig. 4c) are employed. These rings are complex to manufacture, expensive, and have low free volume.

Fig. 4

Saddle Packing

Saddle packing has lower hydraulic resistance than random ring elements and, due to better wettability, provides a larger interfacial contact area between phases. Moreover, in saddle packing, its higher degree of randomness prevents the formation of preferential flow paths (channels) for the liquid, promoting more uniform wetting of the packing.

Fig. 5

Berlie saddles (Fig. 5) have the shape of a hyperbolic paraboloid and are relatively complex to manufacture.

Intalox saddles (Fig. 6) are shaped as a section of a torus and are easier to manufacture than Berlie saddles.

Fig. 6

Block

Block packing is assembled from large-size blocks that have internal channels (Fig. 7). The blocks are made of ceramics, metal, or plastic (Fig. 8). The main advantage of block-type elements over other structured designs is the simplified installation in scrubbers, which is otherwise quite labor-intensive. In terms of performance, block layer is not inferior to other types of structured packing.

Fig. 7

A specific variant of block-type elements is rolled packing. (Fig. 9). It is made from corrugated sheets rolled into a coil. This manufacturing method is simple, inexpensive, and makes it possible to produce cylindrical packing that matches the required column diameter.

Fig. 8

Grid (Chord) and Lump Packing are simpler to manufacture. Grid-type layer consists of a series of lattices made of plates set on edge and is commonly used in cooling towers. As lump-type packing, coke and crushed quartz with particle sizes of 25–100 mm are often used. Grid and lump packings have a relatively small specific surface area.

Fig. 9

TORNADO FB Fluidized Packed Bed Scrubber

Performance:
100 — 175 000 cfm

TORNADO RP Vertical Wet Scrubber

Performance:
600 — 24 000 cfm

TORNADO FB Venturi Packed Bed Scrubber

Performance:
600 — 30 000 cfm

BOREAS-P2 Horizontal Packed Bed Scrubber

Performance:
100 — 175 000 cfm

BOREAS-P3 Horizontal Packed Bed Scrubber

Performance:
100 — 175 000 cfm

TORNADO SP Packed Bed Scrubber

Performance:
600 — 30 000 cfm

Arrangement in Columns

In a mass transfer column (Fig. 10), the packing is placed on support grids. Loading and unloading manholes are provided for placing and removing the elements from the column.

The height of the layer (section or segment) is determined by three factors.

First, uniform liquid distribution over the packing must be ensured. This uniformity is achieved by means of liquid distributors. However, when the height of the layer becomes significant, this uniformity is disturbed — the liquid tends to flow predominantly along the column walls. Therefore, the height of a single layer is usually kept relatively small (3–5 m), and a liquid redistributor is installed after each layer (Fig. 11).

Fig. 10

Second, if the packing is made of brittle materials (such as ceramics or plastics), the lower layers may be crushed under the weight of the packing.

Third, the height of the layer is limited by the mechanical strength of the support grid.

Fig. 11

Materials

The wet scrubber packing material is selected based on chemical resistance, temperature conditions and mechanical strength.

The Torch-Air company's specialists will help to select wet scrubber media that are optimal for specific operating conditions, including aggressive surroundings and high temperatures.

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