technical bulletin 105

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TEX-MESH Technical Bulletin 105

Designing Entrainment Separation Vessels

A knockout drum with a mist eliminator is common whenever a process requires entraineddroplets to be separated from a vapor stream. A simple knockout drum (no mist eliminator) willremove 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 smallerthan 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 (1). A knockout drum with mist eliminator can achieve an

overall efficiency of 99.99% liquid removal.

Knockout Drum Configuration

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), thedrainage 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 coalescer. In this case, the mist

eliminator operates beyond the reentrainment load. Large, coalesced droplets blow off the down stream side of the misteliminator 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 FactorThe key design variable for entrainment separation vessels is a vapor load factor, first derived b Souders and Brown forpredicting flooding in distillation columns (2). The derivation is based o force balance calculation on a droplet falling

through vapor.

The Souders-Brown vapor load factor is:

Fv1 =Vv * (rhov/(rhol-rhov)) ^ 0.5

This vapor load factor is also referred to as a "K" factor for purposes of determining the flux cross section area of a misteliminator or knockout drum. Typically, 0.30 to 0.35 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 variables (molecular weight, pressure, temperature, density, viscosity, surface tension, etc.).This combined variable load factor correlates buoyancy and differential inertial effects for a wide range of liquid/vaporsystems. TEX-MESH Technical Bulletin 102discusses the application of the Souders-Brown transformation for

determining vapor load factor in vapor liquid two phase systems. A similar design variable, designated Fs, is also usedfor liquid/vapor systems. Fs accounts only for vapor inertial effects but not buoyancy effects or differential inertial effects.

Fs is defined as:

Fs =Vv * (rhov )^ 0.5

In hydrocarbon liquid/vapor systems at 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 Stokes's Law as the

nical Bulletin 105 http://www.amistco.com/spanish/tec

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system approaches the critical point. The main reason is that the interfacial tension decreases (approaches zero at thecritical 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 0.30 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 factor of 0.25 to 0.30 ft/sec. They typically operate between

0.20 and 0.35 ft/sec vapor load factor (3,4). I f 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 increasedentrainment 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 knockoutdrum 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, ifthe optimum mum 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 fulldiameter unit, the mist eliminator may be oriented in the longitudinal axis.

For horizontal vessels, the diameter is based on a 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 loading for wire mesh pads inhorizontal flow. The design basis for a 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 followingdesign guidelines are typical:

The top of a horizontal mist eliminator should be at leastone-half vessel diameter from the exit nozzle (top or sidemounted). This reduces non-uniform flow through the padcaused by a radial pressure gradient.

1.

The bottom of a mist eliminator should be at least one vessel

diameter from the centerline of the inlet nozzle (sidemounted). 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 l iquid, one vessel diameter isessential for vapor/liquid disengaging.

2.

The liquid level should be at least one-half vessel diameterbelow the side inlet nozzle centerline in order to avoidinducing entrainment.

3.

If the vessel is to provide a liquid surge volume, theappropriate 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).

4.

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 eliminator 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 forthe 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 slight to100% entrainment.

Entrainment load increases considerably beyond a system load factor of 0.5 ft/sec. Therefore, many designers would


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consider this value to be the upper practical limit of vapor loading in a knockout drum. Since a knockout drum is designedon 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. Thisconfiguration allows for 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 conclusions 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 inmany 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. Thecenterline of a side exit nozzle should be one-half pad diameter above the mist pad. Alternatively, an upward directedelbow 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 (5).

The vessel manway may allow vessel entry below or above the mist eliminator. A manway location below the misteliminator 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 removecollected 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 (Figure 2B) 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 type (6) 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, orcorrugated 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 meshthickness 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, nonfouling, or corrosion. Technical Bulletin 101provides a detailed discussion of selection

criteria for mist eliminators.

Vane Mist Eliminators

The vane type mist pad is also called a parallel plate type or a "chevron" type (7,8). Vane mist eliminators typicallyoperate at higher vapor load factors than wire mesh types because o less susceptibility to flooding. A design K factor o

0.45 ft/sec is typical for vertical upflow (0.65 ft/se for horizontal flow).

Vane mist eliminators are also less susceptible to fouling than wire mesh types. Higher flow rate of drainage liquidprevents adherence of solid particles to the surface of the plates.

The efficiency of vane mist eliminators is less than that for 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 (7,8).


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Vane units may be used in conjunction with a wire mesh pad such as for the 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-MESHTechnical Bulletin 104 discusses design and selection guidelines for vane mist eliminators.

Mist Eliminator Operating EnvelopeThe operating envelopes of the entrainment separation vessel and the mist eliminator should be matched to optimizeefficiency and cost.

Since a mist eliminator functions primarily by inertial impaction (1), higher vapor velocity corresponds to a higherefficiency. Increasing liquid load can induce flooding. Flooding can interfere with entrainment removal even after the upset

subsides and flows return to normal. Eventually, the flood will drain away and the pad will operate properly. Figure.2Ashows 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 curve representing a particular entrainment rate.Once the flood point is reached the pressure drop is not a 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 themeta-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 factorof 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 diminishesconsiderably

Consequently, the efficiency of droplet capture also decreases. For example, the curve in Figure 8 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 theD99 cut-point shifts to 4.7 microns. 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 fulldiameter 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


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ring (typically 2" x 1/4").

The sections are fastened by t ie-wires, "J" bolts, or hold-down bars. The sections are also tie-wired together. The gridson a wire mist pad not only maintain the integrity of the mesh, but also provide support up to a 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 tosupport longer spans of mist pad sections. In some cases, grids may be constructed from heavy-duty metal bars to spanmore 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 about six feet. Dual support rings, hold-down bars, or "J"bolts may be used to secure the sections.

If the knockout drum 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 formounting 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 ProblemsIf specified properly, a mist pad generally operates effectively and is essentially an inconspicuous component in a

process. However, problems are generally the result of fouling (plugging of the mist pad by solid particles). At start-up, ifthe process equipment upstream of the mist pad is not flushed adequately, the mist pad is likely to collect the 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 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 fromunder 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, thena 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 mist pads, but they often cause problems. When a mist pad becomes plugged, eitherthe excess pressure drop indicates the problem, or the 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. Generally, a fouled pad is replaced with a newone.

Non-uniform flow in a mist pad can cause local reentrainment or local inefficiency

If fouling is not present, non-uniform flow is caused by improper placement 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, ifcorrosion 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 (1,3), 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 aknockout drum with a mist eliminator.

Sometimes, a mist eliminator knockout drum is used downstream of a cyclone separator to improve the efficiency ofentrainment separation.


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Electrostatic precipitators are often used to remove small droplets as well as particulates. They are much more costlythan 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 minimize the detrimental effect of entrained liquid in a vapor stream. Veryoften, 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.

References

Capps, R.W., "Properly Specify Wire Mesh Mist Eliminators," Chemical Engineering Progress, December 1994,

pp. 49-55.

1.

Souders, M. and G.G. Brown, "Design of Fractionating Columns," -Ind. -and Eng. Chem., 26:98 (1934).2.

Talavera, PG., "Selecting Gas/Liquid Separators," Hydrocarbon Processing, June 1990. pp. 81-84.3.

Watkins, R.N., "Sizing Separators and Accumulators," Hydrocarbon Processing, November 1967, pp. 253-256.4.

Capps, R.W., "Select the Optimum Pipe Size," Chemical Engineering, July 1995, pp. 128-132.5.

Brinks, J.A., Jr., WE Burggrabe, and L.E. Greenwell, "Mist Eliminators for Sulfuric Acid Plants," Chemical

Engineering Progress, November 1968, pp. 82-86.

6.

Monat, J.P, K.J. McNulty, I.S. Michelson, OX Hansen, "Accurate Evaluation of Chevron Mist Eliminators,"Chemical Engineering Progress, December 1986, pp. 32-39.7.

McNulty, K.J. J.P Monat, OX Hansen, "Performance of Commercial Chevron Mist Eliminators," Chemical

Engineering Progress," May 1987, pp. 48-55.

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technical bulletin 105

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