Packed tower problems

Packed Tower Problems

2. Packed Tower Problems The price we pay for success is the willingness to risk failure. −Phil Jackson, NBA basketball coach I never really liked packed towers, mainly because they cannot be inspected in the same way as trayed towers. Once the packing is installed in a fractionator, foreign objects buried in the packed bed cannot be observed until the tower starts up. Then the buried obstruction manifests itself in an unpleasant manner, meaning, the tower floods and fails to fractionate.

Trayed towers are different. After the trays are installed, I can crawl through every tray and inspect each component for proper installation and cleanliness. Even if there is only a single vessel manway, I can, and often will, check every detail for proper assembly. The one exception is closure of the tray deck manway .

I imagine that packed towers have a potential for greater capacity than trayed towers. But the advantage is small. When a tower is 1 meter or less in ID, the use of trays becomes awkward and packing is preferred. Packing has a lower delta P than trays and hence may be favored for vacuum tower service. For wash oil service (i.e., de-entrainment) and especially in heat transfer pumparound service, I prefer to use structured-type packing. Some services are quite corrosive, and ceramic-type packing is required. However, in normal fractionation service, for new towers in nonvacuum service, the use of packed towers is a poor design practice.

I'm quite sure this statement may be refuted by vendor-published

correlations, which compare tray capacity and efficiency to those of modern types of packing. But these correlations fail to take into account real-world installation, inspection, and fouling problems associated with packed beds.

2.1. Syn Gas Scrubber Flooding In 1972, I was working for American Oil in Chicago. I was assigned to consult on a research project at the University of Chicago. They were developing a coal-to-gas process, and their product gas scrubber was flooding. The scrubber was a 36-inch-ID packed tower. I studied the tower's operation and design for several weeks and issued the following brilliant report: "The scrubber was flooding due to unknown circumstance."

My report was ignored. But when the scrubber was opened later, a plastic bag, which had been used to load the loose packing, was found in the middle of the bed.

A rather similar but more complex problem occurred last year in Lithuania. A fractionation zone in a crude distillation tower was flooding. The fractionation zone consisted of 12 layers of structured-type packing. This is a type of packing that is purchased in blocks. Each block is about 10 inches high, 15 inches wide, and 8 feet long. The packing consists of thin sheets of perforated, crimped metal pressed together to form a block.

A radiation scan (Gamma scan or TruTec scan are common trade names) was performed. A source of radiation is placed on one side of a tower. The percent of absorbed radiation is measured on the opposite side of the tower.

This measurement is done continuously up the length of the tower. Areas of high absorption correspond to a dense liquid phase. Areas of low absorption correspond to the vapor phase. In a packed bed, the transition from a vapor phase to an upper liquid phase determines the elevation in the packing where flooding is initiated.

The radiation scan for the crude tower showed clearly that flooding was initiated between layers seven and eight of the blocks of structured packing.

Obviously, one of two malfunctions had transpired to cause the flooding: An obstruction had been left between the layers of packing.

More likely, as the packing was placed in the tower, someone had stepped

on a thin sheeted block and crushed its upper surface, thus reducing the open area of the packing between layers seven and eight.

I personally supervised the removal of each layer of structured packing.

During my breaks, my wife, Liz, carefully observed the disassembly of the packed bed. As layer number seven was removed to expose the top of layer eight, I found … nothing!

Now what?

Maybe I had misinterpreted the results from the radiation scan as to where the flooding was being initiated. So, I had another, and another, and another layer removed, until I was standing on the packing grid support. Still no sign of any obstruction to vapor flow. Still no explanation as to the cause of the flooding.

It was getting dark and cold. All work had stopped. I had to make a decision.

"Discard the old packing. We'll install all new layers of structured packing," I told the foreman.

"Very well, comrade engineer. But what is wrong with this structured packing?" The foreman had been carefully stacking the blocks of used packing neatly near the tower, as they were removed.

"They are suffering from structural fatigue. Microscopic changes in their pores render them unfit for further service," I explained.

So the new packing, which was identical to the old packing, was installed.

The flooding problem vanished. This incident bothers me to this day. It forms part of my bias against the use of packing in fractionation service.

2.2. Reduction in Percent Open Area Packed beds must be supported. A distillation tray, at least in towers smaller than 10 feet in diameter, can be designed to be self-supporting. Packed beds must be supported on a grid support. This is true for structured packing as well as more conventional dumped or random-type packing.

A typical random-type packing is 1-inch pall rings. Let's assume that the percent open area of this packing is 80%. By the percent open area, I mean:

A single layer of randomly placed rings is floating in the sky.

Sunlight is passing through 80% of the area covered by the rings.

Sunlight is obscured by 20% of the area covered by the rings.

Naturally, the rings can't magically float in the air. They have to be supported by a packing grid support. The openings in the grid support have to be less than 1 inch to prevent the rings from slipping through the grid. If the grid support is constructed from ⅛-inch steel rods, the open area of the grid might be around 75%. Hence, the open area of the packed bed, where the rings contact the support grid, is: (75%) × (80%) = 60% But the grid itself must be supported by a tray ring and a cross I-beam. Let's assume their open area is 90%. Hence, the open area of the packed bed, where the grid is supported, is: (60%) × (90%) = 54% In most process applications, fouling can be expected. In my long and unpleasant experience with packed towers, I have found that these deposits accumulate at the interface between the packing itself and the grid support.

How do I know this?

"Mr. Lieberman, the absorber is clean," said Cathy, the unit engineer. "I washed it with clean, hot steam condensate, even though the packing was clean. I think your theory, that the absorber flooded due to fouling, is wrong.

The packing was clean."

"Cathy, dear girl," I said, "The fouling was iron sulfide (Fe[HS]2). Iron sulfide is not soluble in water. You need to acidify the absorber."

"But the absorber's clean anyway!" Cathy was becoming angry. "I climbed into the top of the tower and inspected it myself."

"But my dear girl, the iron sulfide solids tend to accumulate at the interface between the packing and the grid support. Toward the bottom of the packed bed, where concentrations of hydrogen sulfide in the sour feed gas are greatest."

"I am not your 'dear girl'," Cathy hissed. "How do I acidify the absorber?

Circulate from the top down?"

"No. Circulate from the bottom up. If you do not fill the entire tower with acid solution, the circulating acid will promote channeling. The acid will bypass the most fouled portion of the packing. Then, Cathy, the absorber's vaporliquid contacting efficiency will be degraded."

"What!" Cathy fairly screamed. "Do you have any idea, Lieberman, how much acid it will take to fill my absorber to overflowing? There must be an alternative."

"Look, Cathy. It was you who ignored my advice to use a trayed tower and not packing when you designed this absorber four years ago. And there is an alternative to acid washing the packing."

"Which is?" she asked.

"Take the packing out through the tower top manway in plastic buckets," I responded.

Cathy's beautiful, fair face flushed red with fury as she screamed, "Get out of my office!"

Almost the entire 20-foot packed bed proved to be reasonably clean. It wasn't until the last few feet of packing was removed in the plastic buckets that the packing was found to be mixed with large amounts of black, slippery, iron sulfide corrosion deposits. We spread the packing onto the concrete slab and washed off the iron sulfides with a fire water hose. The rather complex, corrugated grid support was also removed and cleaned.

When the cleaned packing and grid support were replaced, the absorber was returned to service. It flooded far worse than ever. Cathy had all the packing removed in plastic buckets a second time. When I inspected the tower, I saw that the corrugated grid support (see Figure 2-1) had been misassembled after it had been cleaned and replaced. After this malfunction was corrected and the packing was again reloaded, the absorber worked just fine−for a while.

Figure 2-1. A corrugated grid support increases the open area at the bottom of a packed bed.

Cathy, having demonstrated determination in the face of disaster, was promoted to division manager. So all's well that ends well.

2.3. Corrugated Grid Support Packed towers are limited not by the open area of the packing, but by the open area of the interface of the grid support and the packing itself. To reduce this limitation, a corrugated packing support is used, as in Figure 2-1.

If properly designed and installed, the corrugated support can eliminate this capacity pinch-point, unless it fouls. But it's just at this point that fouling deposits tend to accumulate. Also, in larger-diameter towers, the structured support of the corrugated grid may be complicated, and its reinstallation after cleaning, problematic.

There is a reasonable, if not a complete, solution to this malfunction.

Between the grid support and the regular packing, load a layer of larger-size random packing. For example, below a 20-foot bed of 1-inch pall rings, load 1 or 2 feet of 2-inch rings, which have a larger percent open area. Purchase these larger rings with the maximum thickness available. Rings crush rather easily if handled roughly. That's also the reason I avoid aluminum rings.

At the Good Hope Refinery in Louisiana, we experienced continued failure of packed beds of random packing due to the failure of their grid supports. The maintenance manager developed an excellent method to rigidly secure these beds using layers of sturdy grids laid cross-wise and vertical half-inch steel rods. I've given a detailed description of this very successful retrofit in my book, Process Design for Reliable Operations , 3rd edition.

2.4. Packed Bed Failure in a Catacarb Regenerator I'm sitting on the beach in Aruba as I write this story. Six miles away is the idled Valero Refinery where this story unfolded. The packing in the Catacarb Regenerator Tower, according to the plant manager, had disappeared. "How could 15 feet of 2-inch metal rings vanish?" he asked me.

Actually, the packing had not vanished. It had been ground up into tiny metal fragments. Most of these fragments had plugged the shell side of the circulating thermosyphon reboiler (see Chapter 9, "Process Reboilers−Shell and Tube"). The remainder of the broken and ground-up rings were lying in the bottom of the regenerator. What force had ground up these metal rings into such tiny fragments?

My inspections indicated a small portion of the packing support grid had come loose. The rings had drained through this relatively small opening. The circulating catacarb (potassium carbonate solution) had carried the rings into the reboiler. There must have been a channel somewhere in the reboiler bundle large enough for the rings to pass through. The broken bits of rings spun round and round through the reboiler and through the bottom of the regenerator, until they were ground up. Not a dozen intact rings could be found. The cause of the complete loss of the regenerator's stripping efficiency was due to a minor failure to the packing grid support. A similar failure in a trayed tower would have had relatively small consequences and could not have led to a loss in thermosyphon circulation in the regenerator reboiler.

2.5. Bed Hold-Downs Structured packing or grids are often used in the wash oil or de-entrainment sections of vacuum and crude distillation towers. This is an excellent application for such packing, especially when they are constructed of sturdy

layers of grid. The grids, while quite strong, do not weigh very much. A surge of vapor flow may easily dislodge them from their lower support. The liquid distributor above the grid wash oil section may then be damaged by impact with the grid. At the ARCO refinery near Houston, a delayed coker wash oil spray pipe distributor was badly bent upward by such an impact. The resulting unbalanced wash oil distribution flow coked the wash oil grid and turned the heavy coker gas oil product black.

To prevent this sort of upset, a strong hold-down grid placed atop the packing is critical. Sometimes the packing vendors will claim that the weight of the packing will, in itself, be sufficient to resist a pressure surge. This is simply not true. I say this not by calculation, but from unhappy experience.

Always insist that the upward force that the packing hold-down structure must resist must be equal to at least the weight of the grid itself. Drawn from my experiences with this problem at the Good Hope Refinery from 1980 to 1983, I have summarized in Process Design for Reliable Operations

one

practical mechanical design to handle this critical problem.

2.6. Liquid Distribution to Packed Beds Packing is employed in towers in three distinct services: Pumparound (heat removal) (see Chapter 6) Wash oil (de-entrainment) Fractionation (distillation) In wash oil and pumparound services, liquid distribution is accomplished by a spray header. This is a pipe grid. For example, an 8-foot-diameter tower will have a center pipe connected to the inlet nozzle and typically six arms.

Attached to the center pipe and arms are perhaps 15 to 20 spray nozzles.

These are like shower heads, but with no adjustment possible. The standard spray nozzle used in the industry has the following characteristics: 120° spray angle.

Full cone, meaning complete wetting within the spray cone.

Model number corresponds to the maximum free passage of the nozzle.

The term maximum free passage means the maximum-size particle that can pass through the nozzle without plugging the nozzle. For example, if the model number is FMP281, the 281 number means a particle with a maximum dimension of 0.282 inches will likely get stuck in the spray nozzle. This is bad, as it will plug the nozzle. Nozzle plugging is by far the major malfunction encountered with packed beds in pumparound and wash oil service. More on this critical subject later.

The term, full cone is basically a lie. The lie is that the liquid is equally dispersed in the area encompassed by the spray cone. One day, while my wife was away, I removed every drinking glass from the kitchen. I set up a solid array of glasses in my driveway. I tested several reputed 120° full-cone spray nozzles from three different vendors by attaching each nozzle to my garden hose. In all cases, the vast majority of water accumulated in the outer ring of glasses. Admittedly, all my glasses had some water in them, but toward the center, there was very little water accumulation. The least guilty nozzle in this liquid maldistribution problem was the Bete nozzle. So I've always specified Bete nozzles on my designs. But because of this inherent distribution problem, spray nozzles should not be used for fractionation service. This is not just my opinion, but is generally accepted in the hydrocarbon processing industry. For fractionation service, a gravity distributor is required. I'll discuss this in detail later.

The term spray angle is just the angle at which the spray leaves the nozzle.

For example, for the 120° nozzle, the liquid spray angle from vertical is 60°. A wider spray angle increases the wetted perimeter on the packing. However, a wider spray angle may also increase the amount of liquid hitting the vessel wall, which is bad.

2.7. Gravity Distributors Used in Fractionation Service Pilot plant tests conducted by FRI (Fractionation Research Incorporated) have indicated that the ability of any sort of packing−rings, saddles, grids, structured packing, etc.−to fractionate is largely a function of good initial liquid distribution. Tests have shown that packed beds do not redistribute liquid, but instead promote liquid channeling. Finally, spray nozzles do not provide sufficiently dispersed liquid flow, as the liquid is concentrated around the periphery. Therefore, a gravity distributor is required, as shown in Figure 2-2. Liquid is redistributed through progressively smaller and more

numerous orifices. Gravity distributors for larger-diameter towers are very complex and very costly. A properly designed distributor can cost more and take longer to install than the packed bed itself. The most common malfunctions with gravity distributors occur when their various components are not installed level. Or when they plug due to fouling deposits. Or when they are damaged due to pressure surges. Or when they are removed for cleaning and are not reinstalled properly. Or, when designed for a high reflux rate, they are run at a far lower reflux rate. Or when they are poorly designed in the first place. Or …!

Figure 2-2. A three-stage gravity distributor used in fractionation service.

But maybe you have read enough. The point is, it's best not to get involved with complex mechanical features that must function without adjustment inside distillation towers. This is an environment suitable only for rugged, simply designed components, components that need not be precisely aligned and that can withstand fouling, corrosion, pressure surges, and abuse during installation and inspection. Thus, my preference for trays.

2.8. Spray Nozzle Malfunctions I had decided to properly check the spray nozzles used in the wash oil section of Coastal's refinery vacuum tower in Corpus Christi, Texas. I had each nozzle unscrewed for testing using the equipment shown in Figure 2-3. I applied the design water pressure of 15 psig to each nozzle and measured:

Figure 2-3. Testing spray nozzles for spray angle and flow rate.

The water spray angle.

The water flow rate (i.e., the rate of accumulation in the bucket).

Two of the nozzles failed to develop any spray at all. The nozzle internals were missing. Most of the remaining nozzles were partly or totally plugged with green glass from a broken beer bottle. To prevent spray nozzles from plugging, a dual element (duplex) filter is needed. The elements in the filter screen should have one-third the maximum free passage of the nozzles.

Smaller openings will cause the filter to plug too rapidly. Larger openings will result in the nozzle plugging. The one-third value is derived from experience.

That is, the standard one-half value is not small enough. Make sure there are no holes in the filter screen. I mean zero holes! The dual element filter must never, ever, have a bypass. A reasonable fouled delta P before the filter is cleaned is 25 to 30 psi. Filter reassembly should be verified by the unit engineer in writing. At the Coffeyville Refinery where I'm heading as I write these words, I believed the pipefitter's word that a filter was reassembled correctly, and lived to regret my ill-founded trust. Hence, my current return visit.

Spray nozzles may also plug due to a loss of flow. This happens most often

during an electric power failure. A backup source of flushing oil, from an uninterruptable source, is required. This flushing oil must come on automatically on low spray header pressure. Do not use steam as a backup to the normal wash oil flow. Most likely you will wind up with a slug of water when the steam valve is tripped open, which will cause a pressure surge that disrupts the packed beds.

Finally, it's a good practice to observe the spray pattern inside the vessel using water at the intended operating pressure. However, make sure that all piping has been flushed clear before conducting such a test.

2.9. Spray Nozzle Pressure Drop Spray nozzles do not have a very large operating range. If the delta P is less than 5 to 8 psi, a full-cone spray angle will not develop. If the delta P is high (perhaps above 50 psi), the nozzle will form a mist. As the flow through the nozzle increases, the incremental flow will not spray down onto the packed bed below, but instead will form a mist that will entrain to the packed bed above the spray header.

Another problem with a diminished spray angle occurs when using subcooled liquid. This is not a problem I have observed personally. But in theory, the vapor inside the spray cone will condense and cause an area of low pressure to develop inside the cone. This may cause the spray cone to collapse, which ruins the liquid distribution. For this reason, it's best to use saturated, bubble point, hotter liquid in the spray header.

All piping downstream of the filters must be constructed of corrosionresistant steel. Corrosion products which form downstream of the filters are sure to plug the spray nozzles.

On startup, don't be surprised if the filters plug after an hour or less. Scale left from the turnaround has to be flushed out of the system. It's only for a few shifts, or for a few days, that this problem will persist. Make very sure that the operators do not get discouraged and pull the screens out of the filters. Nozzle plugging will surely follow. And you all will understand how I've become so smart on this particular subject.

2.10. Plastic Packing

At the Good Hope Refinery, we melted the plastic or Teflon packing in two absorbers. If you wish to duplicate our achievement, you may select one of the following two methods: Steam out the packing under pressure.

Allow air to enter a packed bed contaminated with Fe(HS)2. The pyrophoric iron will autoignite when dry.

The lesson is, never use a nonmetallic packing in H2S-amine absorption service or in sour water stripper service. Of course, I did not realize that the packing had melted until we started backup and the tower flooded.

Incidentally, I had purchased a large quantity of plastic packing, which I could not decide where I could use after the melting incidents. The plastic rings were dumped onto a big pile in the equipment storage area. Exposed to sunlight, the rings turned into a fine, white powder after a year or so.

2.11. Packed Towers in Offshore Applications There is one area where the use of packed towers in fractionation service appears to have a distinct advantage over trayed towers. That is, on offshore platforms where natural gas condensate is processed. Or, where relatively small amounts of diesel oil are recovered from crude produced on the platform, for use on the platform itself.

I have designed such packed towers using standard correlations. However, the one component that is different for offshore use is the liquid feed and/or the reflux distributor. Assuming the angle of displacement of the tower from vertical is less than 10° due to ocean swells, these specially designed liquid distributors will still produce a normal packing fractionation efficiency.

I did not design the distributor myself, but purchased it as a standard item from Koch-Glitsch. There is a surprising amount of information and published papers on the Internet relating to offshore fractionation technology using packed fractionator towers.

In summary, my negative view of packed towers in fractionation service is entirely a consequence of my many bad experiences. While the theoretical advantages for capacity, and for separation stages per unit of height for

packing versus trays, cannot be denied, experience teaches caution.

Citation EXPORT

Norman P. Lieberman: Process Equipment Malfunctions: Techniques to Identify and Correct Plant Problems. Packed Tower Problems, Chapter (McGraw-Hill Professional, 2011), AccessEngineering

© 2012 The McGraw-Hill Companies. All rights reserved.

Customer Privacy Notice. Any use is subject to the Terms of Use, Privacy Notice and copyright information.

For further information about this site, contact us.

Designed and built using Scolaris by Semantico.

This product incorporates part of the open source Protégé system. Protégé is available at https://protege.stanford.edu//