Mist eliminators

Many separators, both horizontal and vertical, will be equipped with a demisting device located directly upstream of the gas outlet. Its main purpose is to separate liquid droplets from the gas stream prior to leaving the separator. As the last gas-liquid separation step of the separator, it is crucial that the demisting device is designed properly and in conjunction with the other sections of the separator in order to achieve the desired performance.

For optimal separation efficiency, the gas flow should be distributed as evenly as possible across the upstream face of the demisting equipment. Uneven distribution can result in a substantial drop in separation performance and an increase in liquid carryover.

Liquid carryover from a demisting device operating within its designated range generally stems from two factors, which usually occur simultaneously:

1. Droplet capture capabilities of the demisting device

If the mist flow contains droplets smaller than those the demisting device is capable of fully separating, some of the incoming droplets will pass through the device, leading to carryover.

2. Re-entrainment of liquid into the gas within the demisting device

After separation, the liquid remains in contact with the gas flow for a period before it drains from the device’s separation zone. During this time, some of the separated liquid may be re-entrained by the gas and carried to the device's outlet, adding to the carryover.

These two effects counterbalance each other with respect to gas flow. If the gas flow is increased to enhance droplet capture and reduce carryover (factor 1), it also raises the shear forces and increases the rate of re-entrainment (factor 2). Therefore, an optimal operating point or range exists, where the total carryover is minimized, depending on the operating conditions (such as fluid flow rates, properties, and incoming droplet size distribution).

Agglomerators

An agglomerator can be used in addition to the mist eliminator. Agglomerators are typically not required for every application. An agglomerator is typically used in the following situations:

1. When the liquid load to the mist eliminator is high

2. When the expected droplet size is small, for example due to challenging upstream piping conditions

3. When the inlet momentum or inlet gas velocity to the vessel is high.

In the above mentioned situations, the agglomerator has three functions:

1. Pre-separate a portion of the liquid to offload the mist eliminator

2. Agglomerate small droplets to larger droplets which can be separated easier by the mist eliminator

3. Improve the flow distribution to the downstream mist eliminator

Mesh or vanes described below are used as agglomerators.

Mist eliminators

The main types of demisting devices (Refer to [1], [2], and [3]) used in separators are:

1. Mesh pads

2. Vane packs

3. Demisting cyclones

Their main method of separation is direct or inertial interception. For other types of mist eliminators, refer to [4].

Key characteristics of the mist eliminators are compared in Table 1 below.

Table 1: Relative performance characteristics of mist eliminators.

Characteristic	Mesh	Vane	Axial Flow Cyclone
Separation mechanism	Inertial/direct interception	Inertial interception	Inertial interception
Gas capacity	Moderate	High	Very high
Liquid capacity	Moderate	Moderate	High
Turndown gas capacity (max/min)	4	3	3
Fouling tendency (e.g. paraffins, asphaltenes	High	Moderate to high	Low to moderate
Droplet removal range	See Figure 1	See Figure 1	See Figure 1
Pressure drop¹	Low	Low	Moderate

Note 1: For low-pressure drop mist eliminators, flow distribution can be an issue.

Figure 1 - Typical droplet sizes

Mesh pads

A mesh-type mist eliminator consists of a pad made from knitted wires. These wires create a complex path for the gas, causing the entrained liquid droplets to collide with and coalesce on the wires, leading to separation. The liquid that accumulates on the wires drains off and exits the device through gravity. Figure 2 below provides an image of a variety of mesh-type mist eliminators.

Fig. 2—Examples of mesh: stainless steel, polypropylene, and copper (courtesy of Koch Otto York).

Various styles of mesh-type mist eliminators are available, with differences in wire material, wire diameter, void fraction, surface area, and density. There are also co-knit mesh pads, where the wire is co-knitted with materials like Teflon or other plastics for finer droplet separation and use in specialized applications such as amine and glycol contactor towers. Additionally, variable-density mesh pads are available, offering higher capacity and efficiency. These pads feature lower weave density at the bottom (in vertical vessels) to enhance liquid drainage, while the upper section has a higher weave density for improved separation, resulting in overall better performance.

The mesh-type mist eliminator is a highly effective demisting device. Although its droplet capture efficiency decreases slightly with increasing gas density (operating pressure), it still provides excellent separation at all pressures, including high operating pressures.

The gas capacity for mesh pad mist eliminators is typically calculated based on the Souders-Brown equation [3]:

(1)

where

V_g = design gas velocity, m/s

ρ_g = gas-phase density, kg/m³

ρ_l = liquid-phase density, kg/m³

K = mesh capacity factor, m/s.

The recommended value of the K factor varies and depends upon several factors such as:

Liquid viscosity
Surface tension
Liquid loading
Operating pressure

Each manufacturer has its own recommended values. For general sizing, a K value of 0.1 m/s can be used as a guideline. Pressure drops are generally a few inches of water (one inch of water equals 25 Pa.)

Vane packs

Vane packs consist of angled, parallel plates as illustrated in Figure 3. As gas flows through the vane pack, it is forced to change direction several times. Entrained droplets experience inertial forces causing them to impact on the vane blades. Coalesced droplets form a liquid film on the surface of the blades that drains away from the vane pack. How the liquid film drains from the vanes depends upon the type of vane pack as described below.

Figure 3 - A top view of a double pocket vane pack showing the gas flow and liquid drainage channels (courtesy of Jonell Systems).

There are three common types of vane packs: non pocket, single pocket, and double pocket. These three common types are illustrated in Figure 4. These types of vane packs differ mainly in how they manage the flow of the separated liquid or remove it from the separation zone between the plates. Figure 5 below illustrates this process for vertical and horizontal flow pocketed vane packs.

Non-pocket vane packs

In this design, the liquid film flows freely over the plates, driven by the combined effects of gas flow and gravity.

Single pocket vane packs (only for horizontal flow)

This design features "hooks" welded to the plates, intended to restrict the movement of the liquid film caused by the gas flow. The liquid flows down the channels created by the hooks.

Double pocket vane packs

In this design, pockets with openings between the plates guide the liquid film away from the gas flow, reducing the likelihood of re-entrainment.

A non-pocket vane pack can be used in horizontal and vertical flow. When a vane pack is applied as an agglomerator (upstream of another mist eliminator), typically a non-pocket vane is used.

In the double-pocket design, the liquid is more protected from being re-entrained by the gas. A special design of double-pocket vane (hollow pocket vane) also can be used in vertical flow as shown in Figure 5.

Figure 4. - Non pocket, single pocket and double pocket vane packs (courtesy of Kranji Solutions).

Fig. 5—Examples of pocketed or hooked vanes (courtesy of CDS Separation Technologies Inc.).

The separating efficiency of a vane mist extractor depends on:

-Distance between the vanes

-Angle of the vanes

-The number of “turns” in each vane

-Size distribution of the inlet droplets

At higher operating pressures, both droplet capture efficiency and re-entrainment are significantly affected by the physical properties of the gas and liquid. As gas density increases, the device must be designed for lower gas velocities to minimize re-entrainment to account for the increased impact the gas has on the contaminant. However, reducing gas velocity also decreases inertial forces, further decreasing droplet capture efficiency.

Equation 1 can also be used as a guideline for sizing vane-pack areas. Typical K factor values are in the 0.15–0.25-m/s range.

Demisting cyclones

There are generally two types of demisting cyclones: tangential (or reverse) flow cyclones and axial flow cyclones (Figure 6). Axial flow cyclones have the advantage of lower pressure drop and smaller drop size removal.

In the reverse flow cyclone, flow enters tangentially around the gas outlet tube. The flow travels down with liquid being spun to the outer wall and draining out the bottom. The gas reverses direction and flows out the inner tube.

The axial-flow cyclone consists of a tube containing a static swirl element that induces rotation in the gas flow as opposed to a tangential inlet. The liquid film formed on the inside wall is discharged from the tube into a box chamber enclosing the cyclone. The separated liquid is drained from the bottom of the box through a system of drain pipes to the liquid sump in the bottom of the vessel. In addition to the liquid film, some “secondary” purge gas is also removed. For non-recycle axial flow cyclones, the purge gas may need to have entrained drops removed by a mesh pad. In the recycling cyclone, the purge gas is educted back into the center of the cyclone through the stationary turbine. A low pressure region exists because of the spinning flow, similar to that in a tornado. In this way, the purge gas is cleaned again and there is no need for a mesh pad.

Fig. 6—Three types of demisting cyclones (courtesy of CDS Separation Technologies Inc.).

The main advantages of axial flow demisting cyclones (compared with mesh pads and vane packs) are high separation efficiencies at high operating pressures and high gas and liquid capacities. Demisting cyclones have high separation efficiency due to the strong centrifugal forces generated within the device. This efficiency remains high even at elevated operating pressures, although performance may slightly decline with increasing gas density, lower surface tension and low liquid loading.

The pressure drop across demisting cyclones is generally higher than that of mesh-type mist eliminators or vane packs, typically below 80 mbar (1.2 psi).

Mist eliminator orientation:

Mist eliminators can be installed horizontally or vertically, but there are preferred directions depending upon the vessel orientation (Refer to “Separator Types”).

Vertical Vessel

The recommended mist eliminator orientation for a vertical vessel is horizontal for mesh/vanes and vertical for cyclones. A typical vertical scrubber with dual horizontal mist eliminators is shown in Figure 7 showing an inlet device, a mesh agglomerator, and a vane pack mist eliminator. This design has higher turndown and separation efficiency. At low gas rates, the upstream mist eliminator is within its operating range and removes liquid drops. At high gas rates beyond its operating range (i.e. flooded), the upstream mist eliminator acts as an agglomerator, increasing the size of drops that are then removed by the downstream mist eliminator. Typical combinations are mesh with vanes or mesh/vanes with cyclones. If the higher efficiency or turndown is not necessary, a single mist eliminator can be used.

Vertical scrubbers can also have vertical mesh/vane or horizontal cyclone mist eliminator. The inlet can be directly opposite the mist eliminator or below it. However, these designs are known to suffer from poor flow distribution and droplet shattering and are not preferred.

Figure 7 - Vertical scrubber with inlet device, mesh type agglomerator and vane type mist eliminator (courtesy of Kranji Solutions).

Horizontal Vessel

In horizontal vessels, the recommended demisting device orientation is vertical for mesh/vanes and horizontal for cyclones. A horizontal mesh/vane or vertical cyclone mist eliminator is known to suffer from poor flow distribution and is not preferred.

In addition to the demisting device, an agglomerating device can be used. The agglomerator is normally located upstream of the demisting device. Typically, some distance is used between the agglomerator and the mist eliminator. However, in some occasions the agglomerator is located directly upstream of the mist eliminator (no gap in between).

A typical layout based on a vertically oriented agglomerator and mist eliminator is shown in the Figure 8 below.

Figure 8 - Horizontal separator with mesh agglomerator and vane type demister (courtesy of Kranji Solutions)

Nomenclature

$V_{g}$	=	design gas velocity, m/s
$\rho _{g}$	=	gas-phase density, kg/ $m^{3}$
$\rho _{l}$	=	liquid-phase density, kg/ $m^{3}$
K	=	mesh capacity factor, m/s.

References

1. York, O. H. (1954). Performance of wire-mesh demisters. Chemical Engineering Progress, 50(8), pp.421-424

2. Calvert, S. (1975) Entrainment separators for scrubbers, Final Report, EPA-650/2-74-119-b

3. Verlaan, C. (1991) Performance of Novel Mist Eliminators, Thesis, Delft University

4. Green, D.W., and R.H. Perry, Perry’s Chemical Engineers’ Handbook, Ninth Edition, McGraw-Hill, 2018

5. Souders, M and Brown, G.G. (1934), Design of Fractionating Columns, Entrainment and Capacity, Industrial & Engineering Chemistry. 38 (1): 98–103.

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Mist eliminators

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Agglomerators