Vapor Blanketing Reboiler Corrective Actions

Product that crystallized during the shutdown of a distillation column decorate the fouled finned tubes of a reboiter steam-condensate-seal blowing, film boiling, heating-medium inadequacy, boiling-point elevating, foaming, inert blanketing, leaking and undersizing.

Fouling — The vaporization of process fluids at tube walls can precipitate dissolved solids onto the tubes. Tar-like materials in the feed may plate out on metal surfaces. Tube-wall temperatures (which are higher than bulk-fluid temperatures) may polymerize or degrade hydrocarbons, which subsequently settle on tube surfaces. Corrosion products also can foul heat-transfer surfaces. Such occurrences will reduce heat transfer and restrict liquid circulation. (See the example of tube fouling in the photograph to the left)

Vaporization-induced deposition is most severe in kettle reboilers. because they often operate near total vaporization. Fouling is particularly likely if the liquid level drops below the top of the tube bundle. Forced-flow reboilers are the least prone to fouling because they are frequently designed to operate at high liquid circulation rates and a restricting valve or orifice may be installed in the outlet piping to suppress vaporization. The higher fluid velocity also tends to keep deposits from adhering to surfaces.

A drop in the value of a heat-transfer coefficient is a reliable indication of fouling. A gradual rise in the steam condensing pressure (and temperature) needed in steam-heated reboilers to maintain heat input usually suggests the onset of fouling.

Steam-condensate flooding — Condensate backing into an exchanger submerges surface area, reducing the area available for heat transfer. A malfunctioning steam trap (e.g., plugged or incorrectly installed), or high pressure in the condensate-collection header can cause condensate to back up (Figure 3).

One quick check for condensate flooding is to open a condensate blow-

down line upstream of the trap. If steam does not blow through soon, or if the reboiler operation improves, suspect condensate flooding. Abnormal sounds from the trap may also suggest improper operation.

Steam-condensate-seal blowing — If a trap loses its seal, uncondensed steam can blow through the condensate line. This flow can represent a significant fraction of the deliverable steam. The loss often reduces the condensing pressure and temperature of the steam in the reboiler. This problem can be checked by briefly throttling the steam-condensate flow to be sure that an operating seal has been established.

Film boiling — A vapor film blanketing the heat-transfer surface severely reduces the heat-transfer coefficient. This most often occurs when the temperature difference between the process and the heating stream exceeds 100*F. Throttling the steam flow to lower the condensing pressure may reduce the temperature difference and restore normal boiling. Film boiling most commonly occurs at reboiler startups if the process liquid is cold and a rapid startup is attempted with high-temperature

Surging problem plagues all types of natural-circulation reboilers steam or another heating medium.

Inadequate heating medium — Low-pressure steam or a low-temperature heating fluid can reduce heat transfer. Excessively wet steam will increase the quantity of steam condensate that must be removed: the quantity could overwhelm the trap and the condensate piping, backing condensate into the reboiler (Figure 3).

Elevated boiling point — A rise in the boiling point of the process fluid can cause a reboiler temperature pinch. An increase in the column pressure or the depletion of a low-boiling component can elevate the boiling point.

Foaming — Particularly in kettle re-boilers. but also in thermosiphon ones, foaming can produce unstable hydraulics, causing surges, entrainment and poor liquid circulation.

Inert blanketing — This is most often a start-up problem. Inert-gas components can block steam from entering the reboiler and reduce the condensing coefficient by forcing the steam to diffuse through them to reach the tube surfaces. Briefly venting the reboiler steam will remove these components.

Leaks — The process fluid or the

Surging problem plagues all types of natural-circulation reboilers

Vertical thermosiphon

Process fluid

Vertical thermosiphon

Process fluid heating medium can. of course, leak through damaged and corroded tubes, but also at the connection of the tubes to the tubesheet. Obviously, the higher-pressure fluid will leak into the lower-pressure one. but which fluid will be at the higher pressure may differ depending on whether the reboiler is shutdown or in normal operation.

Undersizing — It is possible that the reboiler is limited by heat-transfer surface. A steam-supply control valve that remains fully open may, together with insufficient column boilup, indicate that the heat-transfer surface is inadequate. Fouling, inert blanketing or condensate flooding, which might also be suspected, could be ruled out by checking the exchanger's design.

Thermosiphon reboilers

In addition to the problems common to all reboilers, thermosiphon reboilers have some particular ones, such as those related to liquid level, oscillation, surging, and fluid temperature and distribution.

Liquid level — A low level will diminish the driving force for liquid circulation, reducing the heat-transfer coefficient. For the same heat input, however, the higher vaporization percentage (relative to the circulation rate) and the improved heat-transfer coefficient will, within limits, counter the influence of the lower circulation rate. However, a very low level can change the boiling action to vapor superheating, significantly curtailing heat transfer.

The reduced head due to a low liquid level in the tubes of a reboiler of a

Proc«H fluid

Proc«H fluid

Horizontal thormosiphon

Horizontal thormosiphon

ProMU fluid

Figure 2. Density difference between the liquid entering and the liquid-vapor mixture leaving moves the fluids in natural-circulation reboilers

ProMU fluid

Figure 2. Density difference between the liquid entering and the liquid-vapor mixture leaving moves the fluids in natural-circulation reboilers vacuum column can depress the boiling point of the process fluid. The lower boiling point allows vaporization to begin at a lower tube elevation, increasing the boiling action and the heat-transfer coefficient at the expense of reduced circulation and a higher vaporization percentage. However, the increased vaporization may foul the tubes more quickly.

A high liquid level can also dimin-ishthe performance of a vertical ther-mosiphon reboiler. The greater liquid circulation that results reduces the percentage of liquid that is vaporized, and often causes much of the heat to be transferred as sensible heat This inefficient heat transfer is most significant when the liquid head can be significant relative to the absolute pressure (as in vacuum units), elevating the liquid's boiling point and delaying boiling. Allowing the liquid level to rise above the reboiler discharge into the column can suppress boiling and destabilize the i reboiler hydraul- ;

Upper process- j liquid levels in verti- j cal thermosiphon j reboilers in pres- j sure service typical- ! ly range to the top of the tubes (i.e., to the tubesheet). In vacuum service, liquid levels as low as one-third of the tube length down from the tubesheet are common. Flooding at the reboiler discharge can also result in excessive en-trainment into the trays or packing.

Oscillation — Vertical thermosiphon reboilers can be troubled by oscillation instability caused by the tubeside pressure lagging variations in the tube inlet velocity. Thus, fluctuations in the inlet velocity to the tubes at a certain frequency generate perturbations in the pressure-drop components of the flow-regime zones in the tubes that are out of phase with changes in the inlet velocity. Fluctuations that are 180 degrees out of phase can grow, creating oscillations (at intervals typically of 2 to 10 seconds) in the circulation, steam flow and vapor generation. Although good design can prevent this problem, unanticipated operating conditions can create it. The most frequently recom- i mended remedy is to throttle the liquid I inlet to the reboiler (by means of a | valve or orifice), or to adjust the liquid i level in the column base to a stable operating point.

Surging — This can occur in all natural-circulation reboilers. Sudden, and often violent, waves of vapor followed by a pause in boilup are evidenced by a cycling steam consumption, column pressure drop and base level. The intervals of such cycles range from 30 seconds to several minutes, ana may be irregular (see stripcharts. p. 132).

Surging is common when the column feed contains a high weight fraction of high-boiling components, especially if these boil close to. or higher than, the temperature of the heating medium. In such a case, overheating can deplete the column base of the low-boiling components. After these have been vaporized. a film of high-boiling liquid will cover the reboiler tubes, reducing the heat transfer, with the decline being accompanied by a sharp drop in steam flow and in the circulation of the process fluid. Opening the steam valve further may not produce additional steam flow or boilup.

After boilup stops, the liquid stacked on the column's trays will begin to dump, supplying low-boiling components to the column base, raising the liquid level and displacing some of the high-boiling components from the reboiler. Boiling and circulation will then resume. However, a sudden surge of steam flow and boilup can again quickly deplete the base of the low-boiling components, repeating the cycle.

If a base level-control loop prevents the level from rising to displace the fluid in the temperature-pinched reboiler. the

Steam reboiler may "stall" — i.e.. stop boiling. The dumping of low-boiling components into the column base may lower the base temperature. However, because these components will not enter the re-boiler. circulation will not restart. To combat this reboiler "stalling." first lower the base level, then increase it. so that the reboiler will be replenished with low-boiling liquid and can resume normal operation.

Vaporization-induced fouling can also lead to surging reboiler operation. If the column feed contains a dissolved solid that is being concentrated close to its precipitation point, precipitate may foul tube surfaces or plug tubes, restricting heat transfer to the point that the steam flow falls. With reduced vaporization, solvent-rich liquid dumped to the base of the column may dissolve the precipitates. restoring bottoms-flow circulation. until the cycle repeats.

Liquid temperature and distribution— Poor liquid inlet distribution across a horizontal tube bundle can cause liquid to stagnate and low-boiling components to become depleted in parts of the bundle. The blanketing of the bundle, with the high-boiling liquid and the temperature pinch that results, reduces heat transfer. If the problem is severe, consider retrofitting the reboiler with multiple liquid inlets. Briefly lowering, and then restoring, the liquid level may displace the stagnant liquid.

Kettle reboilers

This reboiler presents problems in vapor disengagement, residue accumulation and level control and distribution.

Vapor disengagement — If head room is not provided in the kettle reboiler for disengaging vapor from the liquid, excessive entrainment can choke

Steam condensate
Figur« 3. Backed-up condensate reduces heat transfer

the vapor return line, causing hydraulic instability and the pressure (and liquid boiling point) in the reboiler to rise. Because the kettle reboilers disengagement space is so much smaller than that in the column, this reboiler is not recommended for boiling foamy liquids.

Residue accumulation — The usuai overflow baffle in a kettle reboiler can trap solids in the shell, fouling tube surfaces. A nozzle for a slip stream can prevent such accumulation.

Level control and distribution — A low liquid level accelerates vaporization-induced fouling of the uncovered tubes. An overflow baffle that sets the liquid level over the tube bundle will prevent this. A high liquid level floods the vapor-disengagement space, causing unstable hydraulics and excessive i backpressure. As in horizontal re-j boilers, poor inlet liquid distribution can result in liquid stagnation and high-t boiling-liquid blanketing of heat-trans-: fer surface area (Figure 4).

i Forced-flow reboilers

; Forced-flow pump circulation suppress-j es vaporization in a reboiler. reducing I fouling. The higher circulation velocity also curtails fouling. Forced-flow reboilers are less sensitive to column liquid level because circulation does not depend on density difference.

Although vaporization is often suppressed by an outlet valve or orifice, a large drop in flowrate (hence, pressure drop) may promote vaporization, which can cause hydraulic problems or vapor bind the heat-transfer area, especially in multiple tube-pass reboilers. If va

Procoss fluid

Procoss fluid

Proeoss fluid

Figure 4. Stagnant regions in a kettle reboiler diminish heat transfer

Proeoss fluid

Figure 4. Stagnant regions in a kettle reboiler diminish heat transfer porization is suppressed, a reduction in flowrate will also boost the process temperature rise, sometimes pinching the temperature driving force for heat transfer.

Overhead condensers

Column overhead vapor is most often condensed by a water- or air-cooled exchanger. Alternatives include the economizer (in which a process stream serves as the coolant), the steam-gener-ating condenser (in which thermosi-phon steam condensate serves as the coolant), ana the heat pump (which provides heat for the reboiler).

Typical condenser problems include: fouling, iow coolant flow, vapor binding. inert blanketing, vent system overloading. condensate flooding, leakage and undersizing.

Fouling — Condensers are less likely to be fouled than reboilers because the mass transfer in the column keeps solids out of the overhead stream. The washing action of the process condensate usually keeps the condenser tubes clean. Corrosion products or material sublimation can cause fouling. Process condensate can freeze if the coolant temperature is too low. such as when a process upset allows high-boiling components to be driven overhead. Such a freeze-up can plug the condensate drain and flood the condenser, coat the tubes and reduce heat transfer, or restrict vapor flow.

Coolant-side fouling is a common problem. Low water velocity, especially on the shellside. can allow silt to accumulate. and a high water-outlet temperature abets mineral deposition on heat-transfer surfaces. Additionally, algae or fungus can foul or plug the exchanger. Regular backwashing can minimize some of these problems. Of course, the extent of fouling can be estimated from the change in the heat-transfer coefficient.

Lou- coolant flow — As mentioned, a rise in the temperature of the outlet water of a water cooled exchanger may indicate reduced coolant flow. A high coolant inlet temperature will, of course, limit condenser capacity. The temperature can vary seasonally, and even with the time of day. particularly with air-cooled condensers. A pooriy located air cooler can recirculate its air.

Coolant

Coolant

Proems* fluid curtailing its capacity, or exhaust its air into another air cooler. limiting the lat-ter's capacity.

Vapor binding

— Vapor or air pockets can block off heat-transfer area from the coolant, especially at startup. This is more likely to occur when the coolant leaves from the bonom of the shell (Figure 5).

Inert blanketing— Inert gases

— such as may flow in through instrument purge lines, and through pipe fittings if the column operates under vacuum. or bepresentin the column feed — can reduce the condenser's heat-transfer coefficient. The problem is more severe near the exchanger outlet, where a large concentration of inert gas can create a mass-transfer resistance to vapor condensation and may even block off the flow of vapors, if the inert gas is not vented. Condensers that are operated flooded for pressure control are prone to accumulating inert gas, if the gas is not shunted via a bypass.

Vent-system overloading — Ifacon-denser becomes overloaded, a backpressure may build up from the vent line or vent condenser, which are usually sized for small loads. A hot vapor flow from an overloaded or poorly performing condenser can be detected by measuring the vented gas temperature.

Condensate flooding — As can steam condensate in reboilers. process condensate can back up into a condenser and cover the heat-transfer surface. Poor baffle orientation in a horizontal condenser can similarly flood the compartments. Excessive entrainment from the column can overwhelm the condenser's process drain, which should be sized for gravity flow to avoid liquid backup.

Leakage — In multiple-tube-pass exchangers. excessive clearance between the tubesheet and the pass-partition

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