Applying general mist eliminator design to a MistFix unit
Although MistFix mist eliminators are cylindrical, they are designed and specified for an application by the same methods as conventional flat mesh and vane units. The overall general procedure is as follows:
1. Estimate the size distribution of mist droplets.
2. Specify the required separation efficiency in terms of the percent of mist to be removed.
3. Considering droplet size, mist load, liquid characteristics, and the characteristics of various mesh types in terms of droplet capture efficiency, corrosion resistance, mist load capacity, and wetability by the mist liquid, tentatively choose a mesh style and material and the radial thickness of the cylindrical mesh pad to achieve the required separation at optimum gas velocity.
4. Choose a design value for the vapor load factor K that is appropriate for the foregoing assumptions, and calculate the optimum design velocity through the mesh using the Souders-Brown equation (explained next).
5. Based on the optimum velocity, the expected volumetric throughput, and the assumed diameter of the MistFix unit, calculate the necessary cross-sectional area for gas flow through the mesh and thus the length of the unit (also explained below).
6. Estimate separation efficiency and pressure drop within the required turndown range.
7. If the estimated results are not acceptable, repeat steps 3 through 6 with a different mesh style, material, or thickness, or with a different diameter of the MistFix unit if that option is available.
8. Check for conformance with internal gas flow guidelines and for height available for the MistFix unit inside the vessel, and revise as necessary.
For explanations of those methods as applied to a broad range of devices, see Amistco’s literature such as “Amistco Mesh & Vane Mist Eliminators,” Bulletin 106.
Regarding Step 3 above, a very wide variety of mesh types are available for conventional pads and MistFix units. Standard wire diameters are 0.011 inch and 0.006 inch. Standard alloys are 304 and 316 SS, but others such as Inconel and Hastelloy are often supplied for certain corrosive services. Plastic monofilaments include polypropylene and Teflon. Yarns co-knit with metal or plastic mesh for capturing very fine mist droplets are commonly provided in Dacron, glass fiber, polypropylene, and Teflon. It may be that the optimum mesh selection is a combination of different types. For instance, an outer layer of fine, dense mesh or co-knit yarn may serve to coalesce very small mist droplets, forming larger entrained droplets that are in turn captured by an inner layer of coarser bare mesh.
Efficiency and pressure-drop estimations (Step 6 above) are beyond the scope of this paper, typically requiring consultation by Amistco engineers. The full spectrum of efficiencies can be provided by a MistFix unit, from a general-purpose efficiency of 99% of 10-micron and larger droplets to a high efficiency of 99.9% of 2-micron and larger droplets. Typically the pressure drop across a MistFix device is 2 to 4 inches of water column.
Step 4 above is conducted for a MistFix unit in the same way as for a conventional flat mist eliminator. These devices are sized for cross-sectional area to achieve an optimum design velocity according to the Souders-Brown vapor load factor K:
The design velocity is the value of the superficial or average gas velocity through the mesh (volumetric flow rate divided by cross-sectional area) that is optimal for the particular liquid and gas involved. It is intended to lie about 10% below the upper end of the operating velocity range. That upper end is the point at which increasing gas velocity begins to cause excessive re-entrainment of captured liquid from the mesh. The lower end of the operating range, in turn, is the point at which decreasing gas velocity begins to cause unacceptably poor droplet capture efficiency.
The Souders-Brown equation allows experimental data taken with air and water on a certain mist eliminator to be generalized to the same type of device with gases and liquids having generally similar characteristics but different densities. Once a design value of the vapor load factor K is established for a mist eliminator type, the design velocity can be calculated for various combinations of gases and liquids.
The appropriate design value of K for a MistFix unit depends on a number of factors that are beyond the scope of this paper. As a first approximation in most cases, however, one can use the figure that is commonly recommended for vertical flat mesh pads: 0.42 feet per second. This K factor corresponds to a velocity of 12 feet per second in the reference case of water and air at room conditions.
It is assumed that the mist load is less than 0.1% volumetric, which is equivalent to 0.5 gpm per square foot at 10 feet per second. Greater mist loads require special considerations.
It is also assumed that the pressure in the vessel is between atmospheric and about 85 psig. The K value should be de-rated by 0.1 (24%) for each 100 psi increase above atmospheric pressure.
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