Gamma scanner helps pinpoint locations of abnormal hydraulics in distillation column
Collapse of trays 16-30
Heavily laden trays with high absorption in vapor spaces
increments of from 0.5 to 2 in. The amount of gamma radiation absorbed and transmitted indicates the quantity and nature of the substance between the emitting source and detector. The gamma absorption of the column wall and insulation is constant except where wall thickness changes. This absorption profile can be translated into a density profile that indicates liquid levels on trays, spray above trays, and clear vapor space. Thus, the density profile reveals the location and severity of en-trainment. downcomer liquid levels, tops and bottoms of packed beds, liquid distribution (packed beds), and the condition of mist eliminators (Figure 6). Liquid weeping has also been detected.
Gamma scanning should not be considered a first step in troubleshooting. It is most useful in confirming or rejecting a diagnosis. The cost of hiring the scanning equipment may be justified by the reduced downtime.
Another radioisotope tool commercially available is the neutron-backscat-ter unit. This handheld device contains a neutron emitter and a detector that registers the amount of radiation re-emitted from a vessel and its contents. Because the isotope can only penetrate
a few inches into a vessel, this method cannot provide a representative measure of the spray height on a tray. However, it quickly and easily detects flooded sections and liquid levels in downcomers. and can serve in deducing where a tray might be missing.
Applying X-ray analysis, which involves a radioactive source and film, to diagnose conditions inside distillation columns has not become common because of the high radiation levels involved and the difficulty of determining which locations to X-ray (Figure 7).
Radioactive-tracer injection represents yet another troubleshooting technique. In it, a radioactive tracer of short half-life is injected into the column. and its dispersement throughout the column and into the product stream is measured. Such things as entrain-ment. tray leakage and fluid maldistribution can be detected by this technique.
Another tool for examining the inside of columns and auxiliary equipment is a video camera having special lenses. The column or equipment must be shut down, but the camera entry saves the time that would be necessary to prepare an enclosure for human entry. Furnished with special lenses, the camera is passed into a column through nozzles. For example, a l%-in.-dia. by 14-in.-long lens is available that will pass through a 2-in. nozzle. The lens provides its own light, and special attachments make possible viewing straight ahead or at angles, because it can be rotated 360 degrees. Although the lenses can be waterproof and purged, they generally are not intrinsically safe. The view is normally clear and sharp, and with good depth of field (Figure 8).
More-specialized lenses are available that will pass through %-in. nozzles. They can be equipped with long lengths of cable so that they can be snaked into locations far away from the opening. Fiber-optic scopes allow entrance through small openings, and can also be snaked into otherwise inaccessible places. Their limitation is the typical 4-in. depth of field (Figure 9).
In addition to detecting and evaluating plugging, corrosion and damage, the video camera has been used to oversee column hydraulic tests (such as fol-
FIQUM 9. CaMe allows camera to be snaked into tight quarters in column
FIQUM 9. CaMe allows camera to be snaked into tight quarters in column
lowing improper feedpipe discharge 1 into open risers on packed-column dis- ! tributors when water was pumped j through the feedpipe). Such viewing is j limited by the fact that entry nozzles may not be available at the suspected problem area.
A full shutdown offers the trouble-shooter the opportunity to view firsthand the physical condition of column internals — to not only search for damage and debris, but to also make sure that the internals were fabricated and installed per the design (See Box, p. 121). Column drawings should be available for checking actual dimensions.
In the case of new columns, this examination should cover such mechanical details as clearances under down-comers. downcomer widths, seal-pan widths, weir heights, tray spacings, tray hole area or valve or cap counts, distributor orifice size and count, vapor-riser heights, and feedpipe details. Internals that are not standard should be regarded as to whether they might hinder liquid and vapor traffic.
In operated columns, look for corroded, damaged or dislocated internals, fouling and debris (Figure 10). Where corrosion is found, the reliability of the structure and the security of nuts and bolts should be ascertained not just visually but also mechanically (by up- | ping with a mallet, for instance). j An inspection can also yield subtle j information about column hydraulic be- j havior and the nature of previous dam- j age. Wash marks on the column wall may indicate spray height. Other discol- j ored areas may point to liquid leakage or j vapor maldistribution. However, make i sure the markings resulted from column ] operation and not from clean-outs. j The nature of the damage may also i provide clues as to its cause. If. for example, tray panels are bent down at their edges, they most likely were dis- i lodged by a force from below, such as | boilup surges or flooding. If the edges i are bent upward, or the tray panels are ' bowed downward, the force was proba- j bly from above, perhaps the result of j too-rapid column drainage, the reboiler heat having been shut off in response to flooding — which allowed the liquid ac cumulation (without the countering vapor pressure drop / to overload the trays, or from a sudden surge of flashed feed.
Failed clamps may suggest the direction of damaging forces (Figure 11). Fouling on trays only below the feed may indicate the precipitation of a material dissolved in the feed. Missing bottom trays may point to operation with high base level or boilup surges.
But even a shutdown inspection has limits. The condition of packing in a bed cannot be determined without the packing being removed, which can be time-consuming and costly. In addition, the steps taken for safe human entry may actually eliminate the cause of a problem. For example, washing solutions for preparing a column for inspection often will dissolve the fouling on a tray or distributor. Care must also be taken to ensure that the inspectors do not bring debris into the column. ■
Mark E. Harrison is a senior chemical engineer with Tennessee Eastman Co. (P.O. Box SU. Kingsport. TN 37662). specializing in the design, troubleshooting; and debottleneckine of distillation columns. Eastman's representative in the Universitv of Texas' Separations Research Program, he has also served as an alternate representative to Fractionation Research. Inc. (FRI). A recipient of B.S. and M.S. • degrees in chemical engineering from Tennessee Technological University, he is a registered engineer in Tennessee and a member of AIChE.
John J. France is Southeast Regional Manager for Glitsch. Inc. (8254 One Calais Ave.. Suite 250. Baton Rouge. LA 70809: tel.: 504-769-76121. He provides technical assistance on distillation systems. A holder of B.S. degrees in education and chemical engineering and an M.S. degree in chemical engineering, all from New Mexico State Universitv. he has worked for
John J. France is Southeast Regional Manager for Glitsch. Inc. (8254 One Calais Ave.. Suite 250. Baton Rouge. LA 70809: tel.: 504-769-76121. He provides technical assistance on distillation systems. A holder of B.S. degrees in education and chemical
Conoco. Inc.. El Paso Natural Gas Co.. and South-em Union Refining Co. A registered engineer in the State of New Mexico, he is a member of the New Mexico Hazardous Waste Management Soc.
Reprinted from CHEMICAL ENGINEERING. Marcn 1989. copyright 1989 By McGraw-Hill. Inc.. witn all rignts reserveo cnGtnŒRinG FCHTURC
Mark E. Harrison,
Tennessee Eastman Co., and John J. France. Glitsch, Inc.*
The typical packed distillation column has one or more beds, each consisting of packing, a support plate, a bed limiter or a nolddown plate, and a liquid distributor. A pipe for introducing feed to the distributor, and other liquid and vapor inlets and outlets. may also be required (Figure 1).
In a packed column, liquid and vapor flow countercurrently and mass is transferred between the two phases continuously. In contrast, mass transfer in a trayed column occurs more-stagewise. as the liquid flowing across a tray comes in contact with vapor rising through the tray, after which the liquid and vapor separate for transport to the next trays.
Gravity is the driving force for the liquid, and pressure differential for the vapor, the latter overcoming the drag resistance of the packing, the liquid and the internal structures. Liquid occupies only as much space as necessary to flow through the packing. The remaining cross-section of the column is available for vapor flow. In effect, the down-comer area is adjustable. This differs from the trayed column, in which down-comer dimensions are fixed.
In packed columns, surface area for heat and mass transfer between the liquid and vapor phases is provided by the liquid film, waves and droplets generated as liquid flows over and through the packing elements (Figure 2). For each column, there are upper and lower limits to liquid and vapor flowrates that ensure satisfactory performance. At liquid rates below 0.5 gpm/ft2 of cross-section, it becomes difficult to distribute liquid
'For author bioerapmes. see the March issue, p. 123.
to the packing uniformly enough to ensure the thorough wetting needed for efficient liquid and vapor contacting. At liquid rates between 25 ana 70 gpm/ ft-. the column is considered liquid loaded and becomes very sensitive to additional liquid or vapor flow.
Vapor flow is bounded on the iow side by that required for vapor distribution.
Understanding how they work is the key to knowing how they go wrong and how to make them right vapor mixing and active contact between liquid and vapor. A vapor rate sufficient to produce a pressure drop greater than 0.1 in./ft of packing is adequate.
With excessive vapor rate, and the corresponding high pressure drop, flooding limits column capacity. A higher vapor rate can increase the drag force of the vapor on the aownflowing liquid to the point at which the vapor begins to resist the liquid downflow. This adds to the liquid holdup on the packing. Liquid entrainment (also called backmixing — i.e.. the recycling of liquid to the packing above), also boosts the net liquid rate and the vapor pressure drop opposing liquid down-flow (Figure 3).
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