A Knockout drum with a mist
eliminator is common whenever a process requires entrained droplets to be
separated from a vapor stream. A simple knockout drum (no mist eliminator) will
remove droplets larger than about 380 microns by gravity settling M. generally
gravity settling removes more than 90% of the liquid entering the vessel
.However the remaining droplets smaller than 380 microns can be a significant
problem for a downstream unit. A mist eliminator in the top of the knockout
drum will remove the remaining droplets down to a diameter of 6 microns or
less, depending on the type of mist eliminator. A knockout drum with mist
eliminator can achieve an overall efficiency of 99.99% liquid removal.
Knockout Drum Configurations
Knockout drums may be oriented
vertically or horizontally. In both types, the mist eliminator may also be
oriented vertically or horizontally. For a vertical mist eliminator (horizontal
vapor flow), the drainage flow is cross-current, whereas for vertical upflow
the drainage flow is counter-current. Because cross-current flow results in
less liquid holdup, a vertical mist eliminator can be operated at a higher
vapor loading without reentrainment (depending on the liquid load and on the
height).
A horizontal entrainment
separation vessel can also be designed to operate as a droplet coalesce. In
this case, the mist eliminator operates beyond the reentrainment load. Large,
coalesced droplets blow off the down stream side of the mist eliminator and either
settle by gravity or are collected by a vane type mist eliminator.
A preliminary analysis may
suggest that a horizontal knockout vessel may reduce cost. In the final
analysis, however, many factors should be evaluated to arrive at the decision
between a horizontal versus a vertical vessel.
Design Load Factor
The key design variable for
entrainment separation vessels is a vapor load factor, first derived by souders
and brown for predicting flooding in distillation columns (2). The derivation
is based on the force balance calculation on a droplet falling through a vapor.
The Souders-brown vapor load
factor is :
Fv1 = Vv *
(ρV/(ρL-ρV))^.5
This vapor load factor is also
referred to as a K factor for purposes of determining the flux cross section
area of a mist eliminator or knockout drum. Typically, .3 to .35 ft/sec is used
as design K factor for entrainment separation vessel.
By expressing vapor loading in
terms of the Souders-Brown transformation, a design variable is created which
is largely independent of the system variable (molecular weight, density,
pressure, temperature, viscosity, surface tension etc). this combined variable
vapor load factor correlates buoyancy and differential inertial effects for a
wide range of liquid/vapor systems. A similar design variable, designated Fs is
also used for Liquid/Vapor systems. Fs accounts for vapor inertial effects but
not buoyancy effects or differential inertial effects.
Fs is defined as :
Fs = Vv * (ρV)^0.5
In hydrocarbon liquid/vapor
systems at a pressures higher than approximately 120 Psia, system load factors
less than 0.35 ft/sec should be used as the design basis. Droplet terminal
velocity departs significantly from Stoke’s Law as the system approaches the
critical point. The main reason is that the interfacial tension decreases
(approaches zero at the critical point ). Another reason is that the density
difference (liquid-vapor) approaches zero.
Knockout Drum Design
A knockout drum (vertical or
horizontal) is typically sized for a system load factor of 0.3 to 0.35 ft/sec.
However, because of the need for a mist eliminator support ring, and because of
roundup to the next standard vessel diameter, vertical knockout drums typically
have a design system load of 0.25 to 0.30 ft/sec. They typically operate
between 0.20 and 0.35 ft/sec vapor load factor (3,4). If other constraints,
such as space or cost, require a smaller vessel diameter, a design load factor
of 0.45 ft/sec may be used; the result is
a reduced margin of safety and an increased entrainment load on the mist
eliminator.
A vapor load factor of 0.2 to
0.35 ft/sec is the optimum range for mist eliminators operating in vertical
upflow. Consequently, a full diameter mist eliminator is usually appropriate
for a vertical knockout drum. However, if the knockout drum is sized according
to a vapor load factor less than 0.2 ft/sec, the mist eliminator should be
sized for optimum efficiency. Consequently, a design other than the typical
full diameter mist eliminator may be appropriate. For example, if the optimum mist
pad diameter is significantly less than the vessel diameter, a sleeve mounting
may be appropriate. On the other hand, if the vessel diameter is too small to
accommodate the required mist eliminator area as a full diameter unit, the mist
eliminator may be oriented in the longitudinal axis.
For horizontal vessels, the
diameter is based on the design factor of 0.35 ft/sec (as for vertical
vessels).
However, because of liquid
holdup, the cross sectional area for vapor flow causes an operating vapor load
factor of 0.4 to 0.5 ft/sec for vertically installed mist eliminators. This
vapor load corresponds to the optimum load for wire mesh pads in the horizontal
flow. The design basis for the horizontal mist eliminator in a horizontal
vessel should be K = 0.5 ft/sec (or less).
The height of a vertical knockout
drum is constrained by a number of factors. The following design guidelines are
typical:
1. The top of a horizontal mist eliminator should
be at least one-half vessel diameter from the exit nozzle (top or side
mounted). This reduces the non-uniform flow through the pad caused by a radial
pressure gradient.
2.
The bottom of a mist eliminator should be at
least one vessel diameter from the centerline of the inlet nozzle (side
mounted). One-half vessel diameter is used in some cases (for light liquid
loading) to satisfy space constraints. However, if the inlet fluid is a
flashing liquid, one vessel diameter is essential for vapor/liquid disengaging.
3.
The liquid level should be at least one-half
vessel diameter below the side inlet nozzle centerline in order to avoid
inducing.
4.
If the vessel is to provide a liquid surge
volume, the appropriate height increment will be required. For preliminary
designs and cost estimates, the vessel aspect ratio (height/diameter) may be estimated
at 2.5 (for zero liquid holdup) or 3.0 (to allow for liquid holdup).
Knockout Drum Operating
Flexibility
Knockout drum turn-down and
surplus capacity (turn-up), result from the two-phase flow characteristics of
the system. The process conditions for most knockout drum and mist eliminators
application occur just below the typical pipe flow regime map. At a system load
factor below approximately 0.5 ft/sec, the two- phase flow regime is
counter-current for the majority of the liquid. At a system load factor around
1.0 ft/sec , the two phase flow regime becomes annular mist flow (for low to
moderate pressure systems). Between 0.5 and 1.0 ft/sec the entrainment load
increases from a slight to 100% entrainment.
Entrainment load increases
considerably beyond a system load factor of 0.5 ft/sec. Therefore many
designers would consider this value to be upper practical limit of vapor
loading in a knockout drum. Since a knockout drum is designed on the basis of
0.3 to 0.35 ft/sec system load factor, there is around 50% to 100% surplus
capacity.
Mist pad flooding typically
occurs around 0.5 to 0.7 ft/sec. Therefore, the practical maximum capacity of
the mist pad/knockout drum combination is again approximately 0.5 ft/sec and
the surplus capacity is about 50%.
Vessel Nozzles and
Internals
A knockout drum typically has a
side entry nozzle the vapor outlet is generally a top exit nozzle. The inlet
nozzle should be located one vessel diameter below the mist pad and one-half
vessel diameter above the normal liquid height. This configuration allows for
the maximum droplet separation by gravity as well as gas jet dispersion and
flow distribution. Straightening vanes have been used to partially deflect the
inlet jet, but no definitive conclusion have been reached concerning the
benefits of straightening vanes in entrainment separation vessels.
In older plants, inlet deflector
baffles were installed in some knockout drums. The idea was to direct the inlet
jet downward and thus to improve the effectiveness of separation. Such a
configuration causes a large pressure drop and in many cases interferes with
entrainment separation because of breaking coalesced droplets into smaller
ones. There is no evidence that an inlet deflector improves performance.
If side exit nozzles are used, a
special arrangement is required to avoid non-uniform flow in the mist
eliminator.The centerline of a side exit nozzle should be one-half pad diameter
above the mist pad. Alternatively an upward directed elbow internal nozzle for
a side exit can be used to promote uniform flow in the mist pad.
Nozzle sizes correspond to the
adjoining pipe size. In the preliminary design of the vessel, the nozzle size
can be estimated by a “quick estimate” method.
The vessel manway may allow
vessel entry below or above the mist eliminator. A manway location below the
mist eliminator is typical. It should be located at 90 degrees from the inlet
jet.
A vortex breaker in the bottom of
the vessel prevents potential pump suction problems if a pump is used to remove
collected liquids.
Tangential entry nozzles have
been used on knockout vessels, but the swirling action of the gas can interfere
with the operation of the mist eliminator. The insertion type unit may be used
with a tangential inlet.
Selecting Mist
Eliminators
The term “mist eliminator” is
used to denote two basic types: the fiber-bed (or candle) type, and the mist
pad (or mesh) type. The fiber-bed is typically a set of cylindrical units which
operates at a lower gas flux (lower system load factor) than the mist pad type.
The mist pad type may be constructed from knitted wire mesh, woven wire mesh,
or corrugated parallel plates. The typical mist pad is an eight inch thick disk
(6 inch mesh thickness plus two inches for grids) which mounts in the bore of a
vessel such as a distillation column or entrainment separation vessel. Typical
mesh thickness varies from 4 inches to 12 inches depending upon the efficiency
required.
Mist pads are manufactured in an
array of unit designs to satisfy a variety of criteria such as maximum
efficiency, pressure drop constraints, non fouling, or corrosion.
Vane Mist Eliminators
The Vane type mist pad is also
called a parallel plate type or a “chevron” type. Vane mist eliminators
typically operate at higher vapor load factors than wire mesh types because of
less susceptibility to flooding. A design K factor of 0.45 ft/sec is typical
for vertical upflow (0.65 ft/sec for horizontal flow).
Vane mist eliminators are also
less susceptible to fouling than wire mesh types. Higher flow rate of drainage
liquid prevents adherence of solid particles to the surface of the plates.
The efficiency of vane mist
eliminator is less than that of wire mesh because of lower surface area per
unit volume (specific surface area). However for many chemical processes the
efficiency is adequate to control entrainment.
Vane Units may be used in
conjunction with wire mesh pad such as for a coalescing knockout drum described
earlier, in which the vane unit is installed downstream of the wire mesh pad.
The opposite configuration (vane unit upstream of the wire mesh pad) may be
used in a fouling service. The vane unit removes the solid particulates (and
larger droplets), whereas the wire mesh unit removes the small droplets.
In general, vane mist pads should
be selected when high liquid rates or high particulate loading are expected.
TEX-MESH Technical bulletin 104 discusses design and selection guidelines for
vane mist eliminators.
Mist Eliminators
Operating Envelope
The operating envelopes of the
entrainment separation vessel and the mist eliminator should be matched to
optimize efficiency and cost.
Since a mist eliminator functions
primarily by inertial impaction, higher vapor velocity corresponds to higher
efficiency. Increasing liquid load can induce flooding. Flooding can interfere
with entrainment removal even after the upset subsides and flow returns to
normal. Eventually, the flood will drain away and the pad will operate
properly. Figure 2A shows the operating envelope of a TEX-MESH TM-1109 mist
eliminator in terms of pressure drop versus system load factor. Below the flood
point the mist eliminator operates along the curves representing a particular
entrainment rate. Once the flood point is reached the pressure drop is not
quite unique function of vapor rate and liquid rate. Furthermore, there is a
hysteresis effect when vapor or liquid rate is reduced. This hysteresis is
believed to be caused by the meta-stable holdup volume in the mist pad matrix.
Figure 2B depicts the efficiency
versus droplet size for a TEX-MESH TM-1109 mist eliminator at the design load
factor of 0.35 ft/sec. At a vapor load greater than the design point, the
cut-point diameter decreases. Likewise for decreased vapor load, the cut-point
droplet size increases. Below about 0.1 ft/sec system load factor, inertial
impaction diminishes considerably.
Consequently, the efficiency of
droplet capture also decreases. For example, the curve in figure 2B has a D99
cut-point of 5.5 microns (99%efficiency at 5.5 microns for 0.35 ft/sec vapor
load factor). For a vapor load factor of 0.5 ft/sec the D99 cut-point shifts to
4.7 micron. For a vapor load factor of 0.1 ft/sec the D99 cut-point shifts to
10.5 microns.
Blanking to adjust
operating range
Because mist eliminators have a
fairly narrow operating range for efficient droplet removal, blanking plates
are sometimes used to increase the flux through an existing mist pad. Often
segmental blanking plates at the sides of a full diameter square mist
eliminator provide operating conditions in the optimum range. For maximum
effectiveness blanking plates may be placed opposite one another on both sides
of the pad.
Mist Pad Mounting
A mist pad is mounted in sections
which are sized to pass through the manway. The sections are supported by a
support ring (typically 2” X ¼”)
The sections are fastened by
tie-wires, “j” bolts, or hold-down bars. The sections are also tie wired
together. The grids on a wire mist pad not only maintain the integrity of the
mesh, but also provide support up to maximum span of about six feet. For plastic
grids, the span should be reduced to about four feet. Support beams across the
vessel are used to support longer spans of mist pad sections. In some cases,
grids may be constructed from heavy-duty metal bars to span more than six feet.
Dual support rings (above and
below) are sometimes used for mounting mist eliminators. In this case, one of
the rings has a removable segment for mounting and demounting the pad.
Vane mist pads do not need grids
because the corrugated plates and tie bolts provide structural rigidity.
However, support beams are still required to support spans longer than six
feet. Dual support rings, held-down bars, or “J” bolts may be used to secure
the sections.
If the knockout is appreciably
larger than the correct diameter for a mist pad, it is often more cost
effective to install the optimum diameter pad than to blank a full diameter
pad. One approach is to install a vertical sleeve for mounting the mist
eliminator. Another approach is to mount a “can” on top of a wide support ring.
TEX-MESH Technical bulletin 103
provides additional details on the installation of mist eliminators.
Operating Problems
If specified properly, a mist pad
generally operates effectively and is essentially an inconspicuous component in
a process. However, problems are generally a result of fouling (plugging of the
mist pad by solid particles). At start-up, if the process equipment upstream of
the mist pad is not flushed adequately, the mist pad is likely to collect dirt,
scale, and other debris.
Furthermore, after the plant has
operated for some time, solids can eventually plug the mist eliminator.
Mist pads are efficient
collectors of solids as well as liquids. If the solids are likely to reach the
mist eliminator, a continuous or intermittent wash system above the pad
establishes counter-current wash flow throughout the pad. Spraying from under
the pad establishes heavy liquid loading at the bottom and a “dry” condition at
the top of the pad. It is critical to limit the total liquid loading (wash
liquid plus entrainment) to about 1.0 gpm/ft2. If higher liquid loading is
unavoidable, then a corresponding decrease in vapor loading is required to
avoid flooding.
Vane mist pads seldom fail
because of fouling. Solids either pass through or are washed off by the coalesced
liquid.
Relief Panels have been installed
in the mist pads, but they often cause problems. When a mist pad becomes
plugged, either the excess pressure drop indicates the problem, or tie wires or
other mechanical supports fail, causing an upset in the process. A fouled pad
is difficult to clean, but it is sometimes done.
Non-Uniform flow in a mist pad
can cause a local re-entrainment or local inefficiency.
If fouling is not present,
non-uniform flow is caused by improper placements of nozzles, baffles or
blanking plates.
Since wire mesh mist eliminators
typically are constructed from stainless steel wire 0.006 to 0.011 inch in
diameter, if corrosion failure is a problem, it will become obvious
immediately. Correct material selection is essential.
Other Entrainment
Separators
A cyclone separator can be used
to collect entrainment, but the efficiency decreases with increasing diameter.
Consequently, at the scale of process plant equipment, the cost and efficiency
often are not competitive compared to a knockout drum with a mist eliminator.
Sometimes, a mist eliminator
knockout drum is used downstream of cyclone separator to improve the efficiency
of entrainment separation.
Electrostatic precipitators are
often used to remove small droplets as well as particulates. They are much more
costly than knockout drum mist eliminators and significantly increase risk of
explosion with combustible materials. For these reasons, a mist eliminator is
often used upstream of an electrostatic precipitator.
Conclusion
The purpose of an entrainment
separator is to maximize the detrimental effect of entrained liquid in a vapor
stream. Very often, a knockout drum with a mist eliminator is the most cost
effective method for entrainment control. Properly designed, the unit will
provide trouble-free performance for many years.
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