The first organic chemical made on a large scale from a petroleum base was isopropyl alcohol, first produced by standard oil of New Jersey in 1920. By this we can understand that the concepts regarding distillation were clear, controlling the unit operations. It is nearly 85 years since then and a lot of research has resulted in not only improving this technology and applying it to the processes, but also has changed completely the vision of this particular unit operation.
It is very important to find the problem than to solve it. Similar is the case regarding distillation and one such problem in distillation is “Component Trapping In distillation Tower”. Column feed often contains components whose boiling points are between those of the light and heavy key components. In some cases the top temperature is too cold and the bottom temperature too hot to allow those components to leave the column as fast as they enter. Water, because of its non-ideal behavior with organics, is a common problem. Having nowhere to go, these components accumulate in the column causing flooding, cycling and slugging. If the intermediate component is water or acidic, it may also cause accelerated corrosion: in refrigerated columns, it may produce hydrates. A large difference between the top and bottom temperature, a large number of components, and high tendencies to form azerotropes or two liquid phases are conducive to intermediate component accumulation.
Intermediate component builds up most frequently takes place over the entire tower, but at times, it is confined to section. Excessively sub cooled feed (or fouling of feed preheater ) can lead to accumulation of an intermediate component between the feed and the bottom. Similarly, excessive preheat may lead to an accumulation between the feed and the tower top.
The accumulation continues until the intermediate component concentrations in the overhead and bottom allow removal of these components at the rate they enter, or until a hydraulic limitation is reached.
The factors governing intermediate component accumulation are essentially those that govern the split of intermediate components between the top and bottom products.
“It isn’t that we can’t see the solution. It is that we can’t see the problem”
INTRODUCTION
Column feed often contains components whose boiling points are between those of the light and heavy key components. In some cases the top temperature is too cold and the bottom temperature too hot to allow those components to leave the column as fast as they enter. Water, because of its non-ideal behavior with organics, is a common problem. Having nowhere to go, these components accumulate in the column causing flooding, cycling and slugging. If the intermediate component is water or acidic, it may also cause accelerated corrosion: in refrigerated columns, it may produce hydrates. A large difference between the top and bottom temperature, a large number of components, and high tendencies to form azerotropes or two liquid phases are conducive to intermediate component accumulation.
Although component accumulation has created major problems in many columns, it has not been extensively addressed in literature.
HOW MUCH ACCUMALATION?
The accumulation continues until the intermediate component concentrations in the overhead and bottom allow removal of these components at the rate they enter, or until a hydraulic limitation is reached.
The factors governing intermediate component accumulation are essentially those that govern the split of intermediate components between the top and bottom products (3):
The VKlK/L ratio. A high ratio near the bottom of the column signifies a large upward movement, whereas a low ratio near the top of the column signifies a large movement downward. When the two come together in the same column, the concentration can be tremendous. Concentrations (‘bulges’) 10 to 100 times the feed concentration are common.
Non-ideal equilibrium. A high activity coefficient at the tower bottom can make an intermediate component particularly volatile. For instance, when the tower bottom is mainly volatile (high VKlK/L). the same organic ca become highly nonvolatile if the top is rich in methanol or ethanol. For a methanol/water separation column, n-butanol in the feed will tend to accumulate to a large extent (2).
The number of stages. Each stage intensifies the upward and downward movement. The more stages the more the intermediate component concentrations toward the middle of the tower.
The concentration of the intermediate component in the feed. The higher the concentration, the greater will be its tendency to concentrate in the tower.
Product specifications. The tighter the specs, the greater the accumulation. Much of this is due to the large number of stages needed to reach the tighter specs, especially stages at high purity, where the upward or downward movement of the intermediate component is intensified.
In accumulation situations, there is always an initial period of non-steady state intermediate component buildup. The buildup tends toward the equilibrium concentration that reinstates the component balance in the tower. This equilibrium concentration, however, may not always be reached, such as when the VKlK/L is high near the bottom and low near the top, and/or the number of stages is high, and/or the concentration of the intermediate component in the feed is high, and/or the product specs are tight. Instead, unsteady-state cycling may set in.
HICCUPS AND CYCLING
A typical symptom of unsteady state accumulation is cycling, slugging. Which tends to be self correcting, the intermediate component builds up in the column over a period of time, typically hours or days. Eventually the column floods or a slug in the offending component exits either from the top or the bottom (the end from which the slug leaves often varies unpredictably). Once a slug leaves, column operation returns to normal over a relatively short period of time, often which minimal operator intervention. The cycle will then repeat itself. Intermediate component accumulation may interfere with the control system. For a instance, a component trapped in the upper part of the column may warm up the control tray. The controller will increase reflux, which pushes the component down. As the component continues accumulating, the control tray will warm up again, a reflux will increase again; eventually, the column will flood. 
Figure 1 illustrates a common configuration of a reboiled deethanizer absorber. The top (absorber) uses a naphthalene oil stream to absorb valuable C3 and C4 components from gas that goes to fuel. The liquid from the absorber is mixed with fresh feed, cooled and flashed. Flash drum vapor is the absorber vapor feed. Flash drum liquid is stripped to remove any absorbed C2 and lighter components. Stripper overhead vapor is also combined with the fresh feed prior to cooling.
When the column is properly designed, all the free water in the feed is removed in the flash drum. Only the minute amount of water dissolved in the hydrocarbon should enter. Inside the tower, the water tends to concentrate. The tower top is too cool, ant the naphtha tend to absorb water, sending it down the tower. Bottom temperature is hot, tending to vaporize the water and send it back up. Overall, the water concentrates in the middle. If the water removal facilities are in adequate, or are not in the region where water concentrates, or the absorber bottoms and/or stripper overhead are internal and not returned to the feed flash drum, hiccups and floods may result.
Means of improving C3 and C4 recovery include cutting boilup, cooling the feed, or increasing the naphtha rate, each of which cools the tower and shifts the water concentration downward. The system in figure had no water removal facilities below the feed. Attempts to recover more C3 and C4 lead to concentration of water below the feed and to hiccups. The hiccups lead to slugs of water in the stripper bottoms, which went into a hot debutanizer downstream, rapidly vaporizing and causing a pressure surge there.
Figure 2 shows a methanol/water system tower. Some of the oil phase from the separator was entrained in the feed, in addition to the oil dissolved in the methanol/water phase. Oil components rapidly become less volatile as they went up the tower, where methanol concentrations were higher, and rapidly became more volatile in the water rich lower sections. Light oil components accumulated, leading to intermittent flooding and hiccups.
Figure 3 illustrates a solvent recovery tower that experienced hiccups even though phase separation did not appear to be an issue. The tower separated a low boiling organics/ water azeotrope from water bottom stream. The tower experienced hiccups, at times due to concentration of a higher boiling component designated. The low boiling organics were very volatile in the water rich layer at the bottom of the column, and became nonvolatile in the cold ethanol rich layer in the upper part of the tower.
Trapping of lights
In many hydrocarbon towers, where water is an impurity, the reflux drum has a boot to remove the water (figure 4). The heavier water phase descends to the boot, from where it is removed, typically on interface level control. If the amount of water is small, an on/off switch is sometimes used. If the boot level control malfunctions, water can be refluxed to the tower, causing fouling and corrosion in the tower.
Plugging may be a problem in the water outlet line from the boot because of low flowrates and because solids and corrosion products tend to become entrapped in the boot and the water stream. The converse problem is leakage rates across the water outlet control valve exceeding the rate of water inflow into the boot. This makes maintaining the level inside the boot difficult and causes loss of product in the water stream. Both the plugging and leakage problems are most trouble some when there is a high pressure difference across the water outlet control valve. A high pressure difference promotes valve leakage; it also tends to keep the valve opening narrow, which promotes plugging.
Both the problems can be over come by adding an external water stream (which may be a circulating stream) to the boot outlet (figure 4). This stream boosts velocity and safeguard against a loss of liquid level. The external water flowrate should be low enough to prevent excessive water back up from over flowing the boot during fluctuations. It is also important to pay attention to good level monitoring.
DIAGNOSING COMPONENT TRAPPING
Key to the diagnosis, especially where hiccups are encountered is the symptom. With hiccups, the symptom is cyclic that tends to be self-correcting, taking place over a long time period. This is seldom less than 1 h, which distinguishes hiccups from other, shorter-period cycles, such as those associated with flooding or hydraulic or control issues, which typically have periods of a few minutes.
Typical cycle periods for component accumulation range from about once every 2 h to once every week. The cycles are often regular, but if the tower feed or product flows and compositions are not steady, the cycles may be irregular.
Drawing internal samples from a tower over the cycle (or over a period of time) is invaluable for diagnosing accumulation. Even a single snapshot analysis can show accumulation of a component. In the tower, a sample drawn from a down-comer was key to the diagnosis. Two snapshot samples had concentrations of the accumulating components (n-propanol and CS) of about 50% , compared to less than 10% in the feed.
Tracking and closely monitoring temperature changes is also invaluable for diagnosis component accumulation. Temperature changes reflect composition changes. Since the accumulation is that of an intermediate key component, it tends to warm the top of the tower and cool the bottom of the tower. This tendency will be countered or augmented by the control system and by the rise in tower pressure drop, and these interactions need to be considered when interpreting temperature trends. In any case, one symptom is common. At the initiation stage, temperature deviations from normal are small, often negligible. As the accumulation proceeds, and the concentration of the intermediate key grows, systematic temperature excursion become apparent. Close to the hiccup point, temperature excursions become large.
The solvent recovery tower in the figure3 was well instrumented, with a temperature indicator every five trays. The tower experienced two types of cycles: a cold cycle, predominantly due to the accumulation of n-propanol, and a warm cycle, mainly due to the accumulation of CS. During the cold cycle, with the control temperature set at 185 degree F and bottom at 215 degree F, temperature below the control tray began to creep down. The deviations were largest on tray20, diminishing towards the bottom of the tower. Over a 2-3 h period, initially the deviations on tray 20 were small, but they became larger. Then, suddenly the column showed flooding signs and the problem by reducing the feed to about 40%, while maintaining the steam flow rate, which allowed the accumulated component to be purged from the top.
During the warm cycle, the temperatures near the bottom rose. The initial rise was slow. Then the bottom temperature suddenly jumped to 230degree F (normal 215 degree F) and, at the same time, the bottom pressure went up by 2-4 psi, indicating flooding. The rise in bottom pressure accounts for much of this boiling point rise (about 3 degree F/psi). this occurred regardless of whether the tower temperature controller was in automatic or manual mode. The overhead temperature went up to about 190 degree F; including that the tower was emptying itself out. At the same time, the bottom flow rate did not change much. The operations tackled that by cutting the steam flow by about half and diverting the bottoms to an 0ff-spec tank. This emptied the accumulation from the bottom rather than from the top, preventing problems in the dehydration system downstream and reducing product losses.
Fig. 5 shows an azerotropic distillation system in which an organic/water azerotrope is dehydrated by injecting a hydrocarbon entrainer. The hydrocarbon is more volatile than the organics, so once the water is gone, it distills up, leaving an organic bottom stream. The water and hydrocarbon leave in the tower overhead, are condensed, then phase separated in the reflux drum, with the hydrocarbon returned to tower and the water (with some organics) removed. In this system, water descended to about Tray 10, forming two liquid phases on the trays above.
The steam flow rate to the re-boiler was temperature controlled. Initially there was a good temperature gap between tray 8 and 12, which is typical of the region where the second liquid phase disappears. As the accumulation proceed, the second liquid phase descended toward Tray 8. It was countered and pushed back by the control action, but later came back. With time, the movement became more intense. In this case, only the temperatures on Trays 8 and 12 were problematic; the temperatures on 4 and 16 did not change much. This is typical and for best diagnostics, the relevant temperatures need to be monitored. In most situations, this is readily achievable even if thermocouples are not present, since with today’s surface pyrometers, column wall temperatures can be reliably measured.
Simulations are also invaluable for diagnosing component accumulation. Steady-state bulges can be readily simulated and recognized on a plot of component concentration against stage number. But since most accumulation problems are non-steady-state, it is necessary to “trick” the simulation to avoid convergence problems (which may reflect the physical reality that the conditions specified do not lead to a stable steady-state solution).
The main trick to overcome this is to study a related system that can converge. One example is to reduce the concentration of accumulating component in the feed to the point where convergence is readily reached. Then the concentration of the accumulating component in the feed is gradually increased, and the changes tracked by means of a tower concentration versus stages diagram for each step.
In the case of a second liquid phase, many stages in the simulation may alternate between a single liquid phase and two liquid phases, making convergence problematic. There either increasing or decreasing the concentration of the second liquid phase can help stabilize the simulation.
Gamma scans and dP measurements are also useful for detecting intermittent flooding. Gamma scans showing flood initiating in mid-column, away from feeds or draw points, provide evidence supporting accumulation, especially if the lower hydraulic loadings are higher near the top or the bottom. Gamma scans taken at different points in the cycle can help trace the accumulation from initiation (at which the trays operate normally) to flood. If enough nozzles are available on the tower, dP transmitters can be just as informative. Finally, sight glasses are extremely useful when safety requirements permit.
REMEDIES
Reducing the column temperature difference
This can be done either by rising the top temperature or lowering the bottom temperature, or both. This enables the accumulating component to escape with a product stream. The effectiveness of this technique may be limited and it can cause off-spec products and/or excessive product losses. A special case is accumulated between the tower feed and the top or between the feed and the bottom. Here, the feed temperature is often lowered to prevent accumulation of the component in the top section or raised to prevent accumulation in the bottom section. Similarly, a feed point change may encourage the component to leave the column at one end or another. Proper bypasses around preheaters and precoolers are invaluable for this purpose.
Removing the component from the tower
Usually, this technique involves drawing a small liquid or vapor side steam from the column, removing the intermediate component from the side stream externally, and returning the purified side stream to the column. If purification is not economical, the side stream may be processed elsewhere, blended with a slop stream, or just purged. The stream drawn should be large enough to remove the amount of the component entering in the feed. Since at he draw-off location the component is normally far more concentrated than in the feed, the stream drawn is usually small.
A typical example of this technique is using an external boot for removing water from inside a hydrocarbon distillation column. Only a small portion of the tray liquid goes to the boot. This portion must be large enough to prevent water accumulation in the tower, but small enough to permit adequate hydrocarbon/water separation in the boot. Proper design of the draw box, as well as the piping to and from the boot, are essential for avoiding siphoning, choked flow, and excessive downcomer backup. As with the reflux drum boot, an external water supply may be desirable. Alternatively, the water/hydrocarbon separation can be performed inside the tower, but this requires a large chimney tray to provide enough settling time for the entire flow. Another typical example is removal of higher boiling alcohol (“fusel oil”) from ethanol/water columns. Unless removed, they will concentrate in the column, and upon reaching their solubility limit from a second phase and cause cyclic flooding as described earlier. Fusel oil is commonly removed by a similar scheme, except that the side stream is usually cooled prior to phase separation and the aqueous phase (rather than the organic phase) is returned to the column.
Another application of this technique is in the separation of ethyl ether from aqueous ethanol, where benzene tends to buildup in the bottom section. Removal of a small benzene-rich side stream out of the bottom section effectively increased the column’s overall capacity.
The component removal technique need not be confined to gravity settling. Other separation techniques, such as stripping and adsorption, may also be employed. the use of a side dryer can prevent hydrates in a C2 splitter.
The best location for the side draw point can be determined by simulation, but changes in composition and simulation inaccuracies can alter the optimum feed point.
Removing the component from the feed
It is surprising how often this can be the best solution. The tower in its initial years of operation, experienced no hiccups. The problem started when the solvent composition changed. During the initial operation, the tower feed had twice the ethanol concentration and half the n-propanol concentration, making n-propanol accumulation far less.
A similar experience occurred in an ethanol/water separation tower that normally received feed from a process that hydrated ethylene and was lean in heavier alcohols (like propanol and butanol). Occasionally, the plant processed a low-cost feedstock from fermentation rich in the heavier alcohols. With this feedstock from fermentation rich in the heavier alcohols. With this feedstock, severe hiccups were experienced.
Modifying the tower and internals
A large number of stages is conductive to accumulation. In one case, doubling the number of separation trays tremendously intensified tower by reducing the number of stages (more reflux, well above the normal optimum), which allowed the same product purities to be maintained.
In some cases, accumulation led to foaming. Addressing the foaming issue (for instance, by enlarging downcomers or injecting antifoam) helped alleviate the adverse effects.
When the main issue with the accumulation is corrosion, changing materials of construction has effectively mitigated the effects in many cases several refinery deethanizers containing stainless steel internals experienced no significant corrosion, whereas those with carbon steel internals often do.
LIVING WITH THE PROBLEM
In many situations, it is uneconomical to cure the problem. At times the product loss is minimal and the problem is just an operating nuisance. Steps such as improving controls, minimizing the accumulating components in the feed by trimming upstream units, reducing or rerouting recycle streams, changing product specs, and providing off-spec and storage facilities have helped to reduce hiccup frequency and intensity and minimize product losses.
CONCLUSION
Regardless of whether you decide to live with an accumulation problem or attempt to solve it, accumulation remains a potential source of erratic operation, flooding, cycling, foaming and corrosion. It is therefore important to understand the principles, be familiar with the industry’s experience, and correctly diagnose and address accumulation each time it appears.
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