Page 278 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

250 Applied Process Design for Chemical and Petrochemical Plants

liquids and systems with very bad fouling conditions.
Table 411 indicates the effect of disengaging height on
the allowable k value. Similar relations should hold for
other mesh densities.

Velocig Limitations

Very low velocities will allow particles to drift through
the mesh and be carried out with the leaving vapor. Also,
very high velocities will carry liquid to the top of the
mesh, establish a “flooding” condition, and then re-
entrain the liquid from the surface of the mesh. For most
situations very good performance can be expected for all
velocities from 30% to 100% of the optimum allowable
design velocity. The minimum allowable safe design veloc-
ity is 10 percent of the value calculated by the equation. ‘0 2 4 6 8 IO 12 14 16 18
The flooding velocity of the mesh is usually about 120 per-
cent to 140 percent of the maximum allowable velocity. Superficial Vapor Velocity, Feet/Second
Generally the maximum allowable velocities are lower Figure 4-18. Typical wire mesh efficiency.
under conditions of pressure, and higher under condi-
tions of vacuum. The limits and ranges of each area being
determined by the relative operating densities of the
vapor and liquid, the nature of the entrainment, and the
degree of separation required.
When the mesh is installed with the pad vertical or
inclined, the maximum allowable velocity is generally used at
0.6’7 times the allowable value for the horizontal position. AirANater System
Ambient Conditions
2.4 meters per second (8 feet per second
Design Velocitj K=.085 (.280)

To allow for surges, variations in liquid load and pecu- 2 3 4 5 6 7 8 9 1 0
liarities in liquid particle size and physical properties, use: Droplet Size (microns)
Figure 4-19. Capture efficiency vs particle size for four types of
DEMlSTER@ knitted mesh mist eliminators. By permission, Otto H.
York Co., Inc.
for the design of new separators. When checking existing
vessels to accept wire mesh, some variation may have to be recovery efficiencies [see Figure 4-19]. Particles smaller
accepted to accommodate the fixed diameter condition, than this usually require two mesh pads or the fiber pack
but this is no great problem since the range of good oper- style discussed later. Carpenter [4,5] shows the calculated
ation is so broad. effect of decreasing particle size on percent entrainment
removed at various linear velocities. For water particles in
Efficien LJ air at atmospheric pressure, the 8p particles are 99 per-
cent removed at 3.5 ft/sec, the 7p at 5 ft/sec, and the 6p
For most applications the efficiency will be 98-99 per- at 6.8 ft/sec. Excellent performance may be obtained in
cent plus as long as the range of operating velocity is most systems for velocities of 30% to 110% of calculated
observed. The typical performance curves for this type of values [35].
material are given in Figures 417B, 418, and 419. For
hydrocarbon liquid-natural gas system, guarantees are
made that not more than 0.1 gallon of liquid will remain Pressure Drop
in the gas stream per million cubic feet of gas. Special
designs using a .?-foot thick pad reduce radioactive Pressure drop through wire mesh units is usually very
entrainment to one part per billion [21]. low, in the order of 1-inch water gauge for a 4inch or 6-
For the average liquid process entrainment the mesh inch thick pad. For most pressure applications this is neg-
will remove particles down to 4 to 6 microns at 95%+ ligible. If solids are present in the particle stream, then