Mist elimination, or the removal of entrained liquid droplets from a vapor stream, is one of the most commonly encountered processes regardless of unit operation. Mesh mist eliminators are fabricated as multiple layers of knitted wire mesh, wherein the crimped strips are stacked to form a pad with a specific density and thickness. Rigidity is provided by a high open area frame, usually metal, consisting of a grid on each side and rods passing through the mesh.
It is a physical separating device that works on the principle of impaction and retention, and is widely used in the industry due to its cost effectiveness and high impaction characteristics for smaller size droplets. The knitted wire media provides surface area for entrained liquid droplets to strike wherein each strand acts as an obstruction around which gas must flow. Within a very short distance upstream of a filament, the gas turns aside sharply but some mist droplets are unable to follow. They strike the filament, adhere, and coalesce to form droplets that are large enough to trickle down and fall away.
Souders-Brown equation is still widely used to the determine the appropriate flow velocity for a given system. The Souders-Brown equation is as below:
The capacity factor is determined through experience and for each application it is influenced by the type and style of mist eliminator, the relative geometry of mist eliminator to vapor flow, as well as process considerations such as operating pressure, fluid viscosities and liquid surface tension. Typical values are 0.35 ft/sec for vertical flow and 0.42 ft/sec for horizontal flow, assuming an air-water system at ambient conditions.
|Style||Density (lb/ft3)||Wire Diameter (inches)||Surface Area (ft2/ft3)|
Mesh mist eliminators are commonly used to solve the problem of liquid entrainment from gas streams. A key limiting factor in the effectiveness of such mist eliminators is drainage. At moderate gas velocities and liquid loads, a thin layer of liquid forms at the bottom of the pad. If velocity or loading increases further, the liquid zone grows thicker, eventually filling the entire pad resulting in liquid carryover in the form of larger droplets. Addressing the drainage issue, AMACS developed a MaxCap® Mist Eliminator which has much higher capacity than conventional 9 lb/ft3 density mesh pads.
The construction features proprietary geometry that enables improved drainage of the captured liquid. This results in higher allowable vapor velocity subsequently offering better impaction and inertial capture of smaller droplets.
A mesh agglomerator is a wire mesh mist eliminator which provides bulk gas/liquid separation and helps coalesce smaller droplets to larger sizes which can be easily separated by downstream demisting device. These are generally employed in combination with a downstream high capacity device such as a pocketed vane or demisting cyclones.
During design conditions, mesh agglomerator operates in flooding conditions and higher velocities favor the droplet capture efficiency thus resulting in agglomeration of very fine droplets. In addition, since the agglomerator is operating in flooding conditions, it has slightly higher pressure drop which in turn helps in distributing the gas evenly. One key advantage of a mesh agglomerator is improving vessel performance during turndown conditions. The low vapor velocity allows the agglomerator to operate as a mist eliminator and provide the required gas/liquid separation.
Our patented MistFix® Insertion Mist Eliminator was developed specifically for retrofit applications where the existing vessel had no mist eliminator support and/or no internal access. We found that the oil and gas industry had a large number of vertical gas Knock Out Drums that were designed to run at relatively low gas rates and had no internals to promote mist elimination. Over time, plant capacities were increased feeding more gas into the drum than was originally considered. This resulted in increased liquid carryover and these empty KO Drums became a bottleneck. The drums required an effective mist eliminator to increase the capacity. Without the MistFix® Mist Eliminator, the only solution is to cut apart and re-weld the vessel shell, which will often require extended down time for field work, post weld heat treatment, re-certification and a re-paint. The MistFix® Mist Eliminator installs without modification to the existing vessel. It drops into the top gas outlet nozzle and hangs from the flange. We recommend a minimum of 8” nozzle diameter. We have used MistFix® Mist Eliminator on both vertical and horizontal drums but note that the gas outlet nozzle must be vertical.
AMACS is the world’s only vertically integrated manufacturer of knitted wire mesh mist eliminators and controls every step of the manufacturing process. These processes include knitting wire, mesh assembly and final custom fabrication of related supports, grids and housings for your mist eliminators or other wire-based column packing process internals. AMACS has an extensive inventory of metallic and non-metallic knitted mesh materials which makes us the industry leader for rush delivery or specialty design of any mist eliminator, knitted or woven wire packing or coalescing filter product requirement.