Columns

Backward from effect to cause

The first step — identifying the problem — will usually be simpler if column performance has suddenly deteriorated than if capacity or efficiency declined gradually. Except when related to auxiliary equipment or instrumentation, most distillation problems arise from flooding (impeded liquid flowpath), excessive pressure drop (impeded vapor flowpath), or inefficiency (insufficient liquid and vapor contact). Of course, these problems do not usually occur independently.

After attempts to stabilize column operation, usually at reduced throughput, the troubleshooter next begins a detailed evaluation of the available information. This involves interviewing operators, reviewing previous problems and their solutions, collecting recorded data, making diagnostic tests, and calculating heat and material balances.

In some cases, the symptoms will strongly indicate a certain problem and cause. At other times, the troubleshooter must consider a series of causes and look for data that support or eliminate them. After having determined the most likely cause, the engineer's next step is to improve performance by modifying the column or its operation, or both. This is usually straightforward once the problem has been identified. However, the engineer must consider alternative remedies, factoring in costs and likelihood of success, and have a backup plan in case of failure. Lastly, the troubleshooter will monitor the changes to ensure that they have been correctly made and have, indeed, improved column performance.

Signs of flooding

Liquid accumulates on trays or in packing if mechanical restrictions or excessive vapor flow (i.e., pressure drop), or both, keep it from passing down through the column (Figure 1). Usually uncontrolled, flooding can stabilize, as when it reaches an area of extra-large tray spacing and sufficient head develops for the liquid to flow down without flooding the next tray. | A column's maximum hydraulic ca-| pacity is the highest throughput at | which it will operate without flooding (Figure 2a). This capacity is a function of the vapor and liquid rates, column geometry, and liquid and vapor properties. The flooding point refers to the location in the column where flooding begins and above which liquid accumulates.

Among the flooding indicators:

• Surges of liquid overhead

• Erratic (including high and rising) pressure drop through the column

• Fluctuating level in column bottom

• A high-temperature profile

• Falling base level or reduced bottoms flow (Figure 2b)

In a flooding column, an overhead surge enters the overhead vapor line, condenser and reflux drum, causing the level in the last to rise abruptly. Reflux to the column cannot reduce the drum level because it is simply recycled overhead. The reflux-plus-distillate flow will be much higher than that expected from normal column boilup.

The liquid accumulation in what would otherwise be vapor space boosts the vapor pressure drop (Figure 2c). As a rough guide, the column is probably flooding if the differential pressure is greater than half the column's height. Below the point of flooding, the pressure drop is likely to be lower than normal because of the lack of liquid downflow.

Flooding usually continues until the operator takes action. However, there are times when a column operates just at the point of flooding, with the normal control variations causing the column to cycle between liquid accumula tion and liquid unloading. This unstable condition, in which flooding appears and disappears, produces an irregular pressure-drop profile. Alternating liquid accumulation and dumping may also cause swings in the base level or the bottoms flowrate.

When the column below the flooding

A systematic procedure can guide you to a speedy diagnosis of the problems that beset distillation columns point is starved for liquid, continued vaporization in the reboiler will cause the base level or bottoms flowrate to fall. Temperatures below the flooding point will be higher than normal because this part of the column will be deprived of liquid downflow containing low-boiling components (Figure 3). In addition, the high pressure drop will elevate the column pressure and. thus, the liquid bubble points.

The loss of stages due to flooding will reduce separation quality. This can be detected as a degradation in the separation of key components or a

For other articles in Chemical Engintmvg on troubleshooting distillation columns, se« "Distillation Coiumn Startup." Nov. U. 1983. p. 221: Troublishootinr Distillation Columns." Juiv 31.1978. p. 70: and "C-iàe to Trouoie-Free Distillation." June 1.1970. p. 139.

cnGinccfiinG remuRc surge of heavier-than-heavy-key components in the distillate. However, the first sign of flooding may be the loss of separation caused by massive en-trainment — often referred to as an "efficiency flood" or a "mass transfer flood." This is most common in columns in which the vapor rate is high and the liquid rate low. as in vacuum towers. An increase in reflux rate may further diminish the separation quality, because the additional entrapment caused by the higher liquid and vapor traffic may more than offset any improvement in fractionation. As flooding conditions are relieved, liquid will begin moving down the column. This downflow can be detected as a hig'n-pressure-drop bulge moving down the column trays. When the bulge reaches the bottom of the column. it can be observed as a surge in base level or bottoms flowrate.

Pressure-drop clues

Excessive pressure drop (higher than design but less than flooding) commonly limits the capacity of vacuum towers, which are usually designed to operate at low pressure drop. If the overall column pressure must be elevated or the top pressure reduced, because of excessive column pressure drop, in order to maintain a certain base pressure, the base temperature may become unacceptably high or the top temperature too low. This can limit the capacity of a column, if the product could be degraded by heat, or if the heat source and heat sink are limited.

Elevated column temperatures, especially at the base, may indicate a higher base pressure. A high base temperature can reduce column capacity by limiting heat exchange in the column reboiler. This will be indicated by high reboiler steam pressure or a wide-open steam-control valve.

If the base temperature is controlled. any increase in base pressure must be compensated for by a hike in the composition of lower-boiling components in the column underflow. If the top temperature is controlled, any reduction in the column top pressure will be compensated for by a boost in the higher-boiling components in the distillate.

Liquid Holdup Trays Distillation Column
FIGURE 2A. Excessive pressure drop represents on* of the surest signs of flooding; 2B. Liquid holdup on upper trays starves bottom of a column; 2C. Rising and erratic pressure drop signals the onset of column flooding

Inefficiency indicators

Poor efficiency usually results from inadequate liquid and vapor contact or insufficient disengagement of liquid from vapor. The first can be attributed to liquid and vapor bypassing each other due to maldistribution, such as by the loss of contacting medium. The key indicators of efficiency problems are off-specification product, excessive reflux and a high rate of boilup.

A drop in column efficiency is often countered by increasing the reflux ratio. If the efficiency deteriorates to the point at which the column approaches the minimum number of stages necessary to achieve the desired separation, a significant increase in reflux ratio and boilup will indicate a problem. This indication may be overlooked if the column continues to make the desired separation. However, the energy penalty and wasted column capacity can be costly.

Now, the detailed evaluation

The operators who are familiar with the column and who watched the problem develop should be consulted. Among the things they can help you determine: When did the trouble start? What were the first signs of it? What was done to correct the problem? Was the process upset prior to when the problem appeared? Did the problem follow any process changes? Have the symptoms appeared before? Are operating procedures being followed?

Always consider that the problem may have occurred before, especially if it is a process, not a mechanical, one. Chronic problems may justify warning indicators, such as feed temperature alarms and composition analysis.

Without information, troubleshooting can only be guesswork. Information sources include operating charts and logbooks, column drawings, piping

and instrument diagrams, and equipment manuals. Charts enable the trou-bleshooter to study the timing of events so as to trace the development of a problem, and to differentiate between automatic controller and operator actions. From the logbook, the trou-bleshooter can spot changes in operating procedures that may have initiated the problem.

Differential-pressure measurements of sections of the column are more useful than a single overall measurement for detecting and locating hydraulic problems. For best results, such measurements should span even- 10 trays or each packed bed. or at least the rectifying and stripping sections. For example, the increase in differential pressure may be several-fold across a flooded 5-ft section of a 20-ft column. The same flooding in a 100-tray column gauged only as an overall pressure drop would be equivalent to an additional 0.3 in. of pressure drop per tray, and might not be perceived as a problem.

Because of the importance of pressure-drop measurements in column troubleshooting, the proper operations of instruments must be verified. When safety considerations permit, manometers or pressure-drop cells can be temporarily rigged between valved nozzles for taking pressure-drop measurements. In some cases, water-filled flexible transparent tubing can serve as a manometer. Such a manometer can also serve as a level sightglass where a permanent one has not been installed.

The troubleshooter should also examine controlled variables and their setpoints, as well as controller outputs and the values of manipulated variables. Also check for improperly ranged instruments. Problems with the condenser, vents and the backpressure controller can be detected via deviations from the normal top pressure. Field pressure gages are notoriously inaccurate, and should be checked with test gages.

With a column temperature profile and temperature measurements of the feed and reflux streams, the trouble-shooter can detect energy balance shifts and such problems as overheated and flashing feeds. Process flows, including reflux, are needed for calculat

Distillation Column Temperature Profile
FKMJRE 3. Flooding distorts the normal column temperature profile

ing material balances. Readings should represent steady-state or averaged values. Controller changes, or changes in column inventory, can skew stream-flow measurements.

A heat-and-material balance furnishes the means for checking the consistency of key data, such as process and utility flows, and product compositions. The data collected should be averaged so as to represent steady-state conditions. Flowmeter readings may need to be corrected for actual fluid densities. Of course, a material balance based on flooded conditions could be in error because of the accumulation in the column.

A component material balance will often pinpoint material-balance problems caused by the ratio of distillate to feed making the desired separation im

Heat Balance Distallation Columns
FKHIftK 4. Corrosion damage can lead to mechanical failure

possible. For example, a column that is supposed to be fed a 50/50 mixture of low/high boilers will not produce a bottoms flow that is free of low-boiling components at a fixed distillate-to-feed ratio of 0.6 when the feed composition shifts to 70/30. If the overall and the component material balances will not close to within 5fc to 10%, one. or more, of the flow or composition measurements is probably inaccurate.

Analyses of the composition of distillate, bottoms, feed and sidedraw streams are particularly useful. The reliability of onstream analyzers, as well as of manual sampling procedures and analyses, must be checked. Random sampling may not disclose some composition problems, such as slugs of water in the feed stream, that may be unsettling the column. A column composition profile, especially in the case of trayed columns, will enable the troubleshooter to evaluate the separation taking place in different sections of the column and to detect buildups of impurities. Recordings of the reflux-drum and column-base levels

can be especially valuable to the trou-bleshooter. as well as of utility flows, and supply and return pressures and temperatures can provide helpful insights.

Besides showing the placement of controi-room and field instruments, piping and instrument diagrams also indicate the location of nozzles and valves that may be suitable for sampling and for serving as alternative feedpoints. However, the troubleshoot-er should not regard such drawings as necessarily representing actual installations. Nevertheless, if mechanical problems are suspected, drawings of the vessel and its internals will provide information on tray or packing assemblies. attachment methods, nozzle locations and special features. Such information can be critical for interpreting data from column instruments and for identifying and visualizing possible mechanical problems.

Operations and maintenance records and files of photographs can be valuable in detecting long-term changes in column performance. A well-documented test run can also be useful, particularly if the file contains — in addition to the usual record of the primary process data — such information as the rectifying and stripping section pressure drops and temperature profiles, re-boiler condensing pressure and temperature. condenser and other exchanger temperature differences and pressure drops, control-valve positions and controller settings. Ideally, the test run had been conducted at normal operating rates, as a capacity or acceptance test.

The column specifications and design calculations will enable the trouble-shooter to compare current with intended operating conditions. Stage and tray hydraulic calculations can provide guidance on the column's limits. Corrosion data, together with the replacement history of column internals, will provide a basis for considering the probability of corrosion damage (Figure 4).

Checking a new column

A new column will sometimes not perform up to operational requirements. The causes may be the result of process-design error —such as the col-

F1GUM S. Multiple pressure taps enable troubleshooter to locate ttoodpoint

Zona with most liquid accumulation

Zona with floodpoint (towast zona» snowing signs of flooding)

Was this article helpful?

0 0

Responses

  • Stefan
    What is the importance of temperature indicators at different trays of distillation column?
    7 years ago
  • dominik
    What does an increasing differential pressure in a distillation column indicate?
    6 years ago
  • trahand
    What is "underflow" in a distillation column?
    2 months ago

Post a comment