Localized Wear and/or Solids Buildup
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If upon inspection, unusual wear and/or solids build-up is noticed, it is suggested that those areas be documented (photographed or otherwise noted). Often, localized wear is a function of poor gas distribution that may be a function of the operation of the tray below the problem tray. In other words, once a stream of higher velocity gas is established within the scrubber, there may not be enough "natural" correction forces available in the scrubber to make a correction. The tray scrubber design usually avoids the use of baffles, etc., to provide specific gas flow patterns. The assumption at the design stage was likely uniform "plug" gas flow. The gas pattern was assumed to be uniform across the face of the tray (or trays). High local wear or solids build-up usually indicates a poor distribution of gases. Corrective methods often include investigating the liquid distribution on the "problem" tray and the tray (if any) above that problem tray. As in "a" above, the tray is usually checked to make certain the tray is level and, if level, a study is made as to how the liquid is distributed across the tray. Sometimes solids can build up in the liquid entrance weir (where the recycle liquid enters and is distributed) thus causing poor liquid distribution from the start. Cleaning that liquid entrance weir box area can often solve the problem. If wear is noted at the tray perforations, excessive "normal" gas velocity may be the cause. The tray supplier could possibly recommend trays with large perforations but would need to tune the tray selection to the limits of pressure drop. If the application inherently shows excessive wear, a pre-scrubber such as a venturi type may be needed. That device would reduce the gas inlet particulate loading to reduce the amount of particulate entering the tray scrubber. It is not unusually to see venturi scrubbers being installed as a "team-mate" for a tray scrubber. Another wear producer is operating the tray scrubber at excessive recycle solids content. Usually, a limit of about 1% suspended solids (by weight) is applied to a tray scrubber recycle liquid circuit.
Tray scrubbers operate in a range between "weeping" (wherein the gas velocity is lower than that required to keep the liquid above the tray) to "flooding" wherein the gas velocity is so high that the liquid is ejected "overhead" (either upward to the next tray or upward to the droplet separation stage). Weeping can occur (unless desired with a weeping sieve tray) if the gas velocity (in effect kinetic energy) falls below the velocity through the perforations required to hold up the liquid. Obviously, the size of the perforations and the gas density greatly affect the required kinetic energy. If a tray weeps, then perhaps smaller diameter perforations may be required, or blank offs can be applied to the lower surface of the tray. The tray supplier could possibly recommend and supply replacement trays that resist weeping. If blank offs are contemplated, one must be certain that the installation of the blank offs does not adversely affect the gas flow pattern. A blank off applied to a lower tray in a multiple tray scrubber may upset the gas flow through not only the tray in question but also the ones above that tray. Usually, however, the amount of blanking is minor (often less than 10% of the tray) to increase the gas velocity through the perforations of the remaining open portions of the tray to reduce or eliminate weeping.
Sudden Pressure Drop Changes
With a tray scrubber, a sudden reduction in pressure drop usually occurs wherein the "downcomer" seal located ahead of a tray is no longer able to seal. The overall scrubber pressure drop will decrease as the resistance to flow across a tray is reduced. The downcomer seal is in effect a liquid trap (much like an under-sink trap) that normally prevents gases from sneaking up the downcomer without passing through the tray. If the application gas density or gas flow rate increases, the seal height required may be insufficient. Often, adding some height to the seal trap can recover the required seal depth. Also, under scheduled maintenance, the downcomer area(s) should be cleaned (but are often overlooked since they can be difficult to inspect). That seal area can usually be flushed, or power washed without complete inspection. Another cause of a reduction in pressure drop can be the corrosive failure of a tray. If a hole develops in a tray, both the resistance to flow caused by the gas and that of the liquid occurs at the same time. The hole need not be exceptionally large to cause a significant pressure drop reduction. Obviously if an inspection reveals a damaged tray then that tray should be replaced. Yet another cause of a change in pressure drop could be the result of a flooding incident in the scrubber. The trays are typically either bolted in place or wedges are used to secure the tray. If flooding (even for a short period of time) occurs, the forces imparted by the surging liquid can be quite violent causing a tray (or trays to become dislodged. One moved, the tray may not return to its original and proper) location. If the tray is not damaged, the tray can often be returned to its proper position. If the tray is no longer flat, the tray would need to be returned to a flat plain or be replaced. A sudden increase in pressure drop usually is a result of a "puff" of solids from the process blinding the lower surface of the tray. Once the perforations are plugged, many tray scrubbers (unless they are equipped with lower tray "face" sprays that spray upward to clean that tray surface) lack any mechanism to reduce plugging. Given a drawing of the scrubber, it may be possible to add face sprays to an existing tray scrubber. The water balance would need to be checked however to avoid flooding of the lower tray since some of the face spray liquid will go upward through the tray perforations and add to the inventory of the liquid above that tray and perhaps increase that tray's pressure drop. Excessive solids in the recycle liquid can also increase the pressure drop.
Sudden Changes in Efficiency
A reduction in efficiency (absorption, cooling, particulate removal) could be caused by any (or even all) of the above. The preceding are "mechanical" type effects whereas the overall performance of a tray scrubber may be a combination of both mechanical and chemical actions. If the scrubber is used for absorption followed by neutralization or oxidation of absorbed contaminants in the liquid phase, adjustments to the chemistry of the recycle stream may be required. These adjustments are like those that can be applied to packed towers therefore the reader is directed to that section. Another thing that can occur with a tray scrubber that can reduce the efficiency is when the gas velocity is increased to a point wherein the liquid on the tray goes "overhead". Instead of the liquid going across the tray and down (counterflow) the liquid can be carried upward (to the next tray above or even out to the droplet eliminator). The tray may not be flooding however the distance between trays is such that the liquid cannot separate and fall back but rather be carried up into the upper tray. When that occurs, the liquid above is now contaminated with the liquid from below (dissolved solids, temperature, etc.) thus stage wise separation is reduced. The tray spacing however is usually a "given" and is an inherent part of the vessel tray spacing, thus the optimization techniques are limited. Perhaps larger tray perforations could be used (to reduce the kinetic energy of the gas thereby reducing the height to which the liquid can rise) or the amount of gases being delivered to the scrubber can be reduced (such as precooling and condensing the gases ahead to that scrubber).
If the tray scrubber is applied to a process that emits particulate, the scrubber likely is equipped with a "face spray" as mentioned above. The face spray area usually features full cone spray nozzles whose pattern is selected to completely wash the lower portion "face" of that tray. If possible, that spray circuit should use clean water but too often it does not. If build up is experienced, sometimes the spray circuit solids content can be "cut" by blending in fresh (make up) water to that header circuit rather than adding make up water to the scrubber sump. The fresh water would therefore be put to beneficial use prior to draining to the sump where it would end up anyway.
Droplet Eliminator Effects
Ineffective draining of the droplet control device can cause poor droplet separation. Tray scrubbers often are equipped with "vane type" droplet eliminators. These look like large turbine blade assemblies. These devices use near radially oriented vanes (the vanes are installed tangent to a central cylindrical core) that impart a centrifugal action to the rising gas stream. That action forces the entrained droplets towards the vessel wall from which point the droplets drain through traps to the tray below (or sometimes out of the vessel). If those traps become plugged, the peripheral zone near the vessel wall becomes flooded and re-entrainment of liquid can occur. To optimize the droplet control, these traps must be kept free flowing. Some scrubbers use a timed spray of fresh water to periodically spray the vanes and thus momentarily flush the drains as a purge. This is essentially a clean in place type procedure that supplements periodic maintenance. Of course, if the vanes become damaged that droplet separation could be reduced. If the droplet separation cannot be easily improved through vessel repair and/or cleaning, then a supplemental cross flow droplet separator could be a possible remedy (see other sections where this technique is described).