September Process Puzzler: Uncovering cause of solid build-up

Readers offer suggestions to improve reliability of a vibrating screen in Chemical Processing's monthly Process Puzzler feature.

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Question from July's Chemical Processing

A vibrating screen separates pulverized lumps of coal from dust (Figure 1). The screen is fed from a lock-hopper above. The fine particles must be separated because they could become carry-over for a gasifier (entrained-flow); a pelletizer scheme is being tested to re-use this material. The lumps are then fed to a screw conveyor to the gasifier. On a vibrating screen, solid is supposed to be spread evenly across a mesh screen. The rocking motion and the slight slope cause the solid to flow towards the discharge and into either the pneumatic conveyor pick-up pipe or the hopper for the screw conveyor. This isn’t happening. Instead, coal is building up on the screen and blocking flow from the lock-hopper above. The screw conveyor hopper is plugged and the pick-up pipe is plugged. Adjustments to the slope and the rocking motion have been increased to no avail. Any suggestions?

Figure 1. Flow is blocked at the discharge of the vibrating screen.
Figure 1. Flow is blocked at the discharge of the vibrating screen.

 

Modify the screen feed

I've seen similar problems on other screeners, especially the models that "rock" rather than vibrate. It was a little confusing how the screw and pick-up pipe could both plug if the screener is being blocked by a build-up. However, if I assume that the screen size has been chosen correctly and isn’t broken, the conclusion would be that the coal particles are wedging themselves in the openings and accumulating. This build-up will sluff off periodically causing plugging. When the sluff occurs, large coal particles come crashing down on the screw causing a bridge to form or overloading the screw. The fines are released in clumps, as well, and fall into the pick-up hopper, which also gets overloaded. This should be easy to observe and report back. Most likely, the release of solids from the lock hopper is the main cause of the overload. This can be solved by placing a sheet of metal on top of the feed portion of the screening deck. Sometimes a perforated plate can be used if the amount of fines is very heavy. The plate will spread the flow of solids over the screen and provide a lateral velocity that will keep the larger coal pieces from sticking on the screen. The fines will settle onto the surface of the plate and gently fall through the screen; eliminating the clumps to the pick-up pipe.

Tom Blackwood, director of technology
Healthsite Associates LLC, Ballwin, Mo.

Replace the valves

I see two problems: surge flow from the lock hopper valves and poor feeding at the discharge of the vibrating screen. Obviously costs must be a consideration, especially downtime. First check the design calculations, if possible, for flow from the vibrating screen. Next, inspect the discharge of the vibrating screen feeding the hopper below and the pick-up point for the pneumatic conveying system. Look at the bottom part of the hopper connection spool checking for the angle and for dead space where solids could bridge. If the inspection confirms problems, consider modifying the hopper connecting spool first to increase the angle of repose and reduce dead space (Figure 2).

Figure 2. Replace lock hopper valves with a feeder screw conveyor.
Figure 2. Replace lock hopper valves with a feeder screw conveyor.

The angle for the vibrating screen outlet should be at least 70o from horizontal. Since the physical properties of the coal may be different than those developed for the design re-run the Jenike test. This test was developed to measure the internal and wall friction for solids in silos (refer to: http://www.engineering.uakron.edu/~chem/fclty/chase/Solids/SolidsNotes10%20Hopper%20Design.pdf).

If modifying the hopper fails, consider a more expensive modification of the process. If there is sufficient room, replace the lock hopper valves with a feeder screw conveyor. Sometimes, lock hopper valves are installed because there is a space restriction; if so, consider deflector plates to reduce dust formation or a side-entry feed to reduce the height of the screw conveyor.

The feeder should use a variable pitch or conical screw to provide even flow from the bottom of the mill storage tank. A conical screw will have a higher torque and cost. The enclosure should be tight and other precautions e.g., nitrogen purge, are necessary to avoid the dangers of an explosive dust. Off-set bearings may perform better with fine dust and make sure grease fittings are accessible and bearings are greased daily.

I believe that fine dust is being pulled into the coal hopper below the vibrating screen. Depending on the pull of the vacuum system, the pneumatic conveying system could be creating negative pressure inside the vibrating screen. This could be holding the coal to the screen and contributing to plugging the line. It is probably not possible to reduce the vacuum from the system, so the flow of solids must be controlled.

To improve the feeding of the pneumatic conveyor from the vibrating screen, add a rotary air lock, or rotary valve (RV) and a standard-pitch screw conveyor. You may want to consider a nitrogen purge here, depending on the clearance and design of the RV and screw. Because of the nature of the dust, the enclosure must be tight. Flow of the coal to the hopper can also be improved.

Install an enlarged funnel-type spool between the vibrating screen and the hopper. Care must be taken because of the large mouth, which could tend to leak. Reduce the gap as much as possible and check the tramp air loading on the vacuum system. Feed from the hopper should be verified. Confirm that the feed screw below the hopper is a variable pitch design.

I recommend two final improvements. Install vibrators (not an air cannon) at the bottom and side of the hopper and at the bottom and side of the pulverizer hopper. Avoid using these flow aid devices except during start-up. Since no clean-out ports were designed, it would be wise to install pairs (180° apart) on either side of RVs and at the bottom of all hoppers. Check the air loads on the pneumatic conveyor: ports may leak.

Prabin Shrestha, energy engineer
ROQUETTE America, Keokuk, Iowa

Look at the material balance

Based on the problem description the system is under-designed. Combination of the following is possible cause of the problem: 1) coal feed rate is too high for the size of vibrating screen; 2) as the coal moves, further size reduction is generating additional dust; 3) not enough air is being used to remove dust from the system; 4) air impingement nozzles aren’t properly placed; and 5) dust-bridging is leading to plugging in the hopper.

Coal build up on the screen may suggest that the there is insufficient slope for the coal to move forward.

If the figure is correct, the orientation of the dust intake may be wrong. Pick-up pipe should be straight up so that all the dust can be directed to the dust collector without any accumulation or bridging in the pipe. Air has to impinge at high enough velocity to dislodge the dust and carry it to the pick-up pipe. In addition, it might be necessary to have air flow countercurrent to the coal flow along the vibrating screen bed so that all the dust goes to the dust collector with a minimum of dust entering the hopper.

The system should be redesigned with a good mass balance. I believe a proper design with above considerations can solve the problem.

Girish Malhotra, president
EPCOT International, Pepper Pike, Ohio

Add vent lines

I’m not sure how to resolve the problem with the dust but have some thoughts on avoiding plugging in the coal. I believe the symptoms can be solved by adding some additional lines. The following problem being faced in the system: the feed line to vibrating screen is getting choked; the material isn’t sufficiently flowing to screw conveyor; and, sometimes the line to screw conveyor is getting plugged.

Figure 3 describes the changes that I recommend. There are several solutions that should be considered separately.

Figure 3. Flow is blocked at discharge of vibrating screen.
Figure 3. Flow is blocked at discharge of vibrating screen.

To avoid choking of feed line to vibrating screen, the gap between line and screen should be increased. If there is a cloth bellow at the end of the pipe, its length should be reduced so that there will be sufficient gap between it and the screen.

The counter weights of the motor of the screen should be adjusted to increase the vibration in horizontal and/or vertical direction. It may be that the vibration motion is inadequate for the task.

To avoid air-locking of the rotary valve, a vent line should be installed connecting the rotary valve vent port with the top of the feed hopper. This line is shown in the drawing. Take care with this line that there is sufficient slope to avoid build-up of solids. Piping should be flanged, not welded, for ease of disassembly. Adding view-ports to such lines might be useful. Size the vent line based on the volume discharge rate for the rotary air lock.

Watch the process after making these improvements. If the problems remain, an additional vent line can be provided with a removable screen. This line will vent only air and solid won’t plug the line. Piping should be designed for disassembly. Use the leakage rate for the rotary valve for sizing. To avoid plugging of the feed line to screw conveyor, add a vent line to the screw conveyor connecting it to the feed hopper; the cross-sectional area of the vent line for screw conveyor should be the same as the area for the screened line.

If these pipe changes don’t effect an improvement, consider re-evaluating your equipment choices.

Kamal Parikh, general manager, technical service
Reliance Industries Ltd., Surat, India

Fine-tune the screen

With this type of equipment it is difficult to develop an understanding of the solid flow problem without seeing the equipment in motion. I would look for ways to impart energy perpendicular to the screen to dislodge particles and prevent blinding. One idea might be to increase the angle of the screen. This could reduce residence time and prevent backup into the feeder. Another idea is to increase the screen mesh size. This may reduce blinding, depending on the particle size distribution. The drawback, obviously, would be increased particle loss — to the dust collection system. After the change to the material balance is established, look at the cost — both for disposal cost (of the dust) and the loss of product.

In our plant, we use a vendor’s product that includes rubber balls in its vibration screen. The balls bounce up against the underside of the screen to keep it from blinding off. The balls come in different materials that impart varying amount of force. They need to be inspected for wear periodically and replaced as needed. If adjusting the screen doesn’t work out, consider this type of screen, if temperatures allow.

Tim Bonner, quality engineer
Celanese, Calvert City, Ky.

It’s a moisture problem

Based on your info and the fact that upstream and downstream conveying also is plugging up; the moisture content of the coal is probably too high for reliable handling. Obviously then a drying step is needed upstream.

Assuming this costly configuration change is a last resort, the screening problem could be helped greatly by using higher G force equipment (approaching 5 Gs). Many screen de-blinding options are available from vendors.

Also the “rocking motion,” as described in the puzzle, isn’t ideal for this application. If the problem is moisture, you need a “neck snapping” action to dislodge damp materials from the mesh. Options include ball trays, self-cleaning kits, high-frequency screen energizers, and high-G-force separators. Another alternative may be a different mesh design. It is also possible that a material incompatibility is contributing to the problem by affecting surface roughness, for example.

I recommend a site visit by the vendor engineer, or its competitors. You should request a laboratory test of the coal, from various stages of the process and an inspection report. Hopefully, the vendor can reduce your problem by making a few adjustments in the vibrating screen and recommending minor improvements in the system without adding the drying step.

Peter Knox, global applications specialist
Sweco Florence, Ky.

Swap out the wire mesh with V-wire

I think a change in the wire screen size or type may improve flow. Wire screens aren’t flat — coal may not move smoothly across the surface. The simplest remedy may be to use v-wire screens instead of standard wire mesh. V-wire is very smooth with slot openings instead of knitted wire. This allows material to move, particularly if the lines of the V-wire are oriented in the direction of the desired flow. They’re also very strong, so that maintenance may be reduced over a standard screen, though the initial cost is likely to be higher.

The question is: can we salvage the vibrating screen? The frame that holds the screen may be reusable to hold the V-wire. That way, no changes to the mechanical tilter and shaker would be required. V-wire obviously weighs more than a standard screen, so a mechanical engineer would need to check to see that the machine can handle the additional weight and strain on the other parts, assemblies, bearings, etc. If the rocker arms and such are too light, a new unit altogether may be required.

Buying a new vibrating screen, one that can use the V-wire, is a last resort. There are a few quick-fixes that may improve matters. Does the existing vibrating screen have an option for rotary motion in addition to standard vertical vibration? I've seen standard up and down vibration fail to solve solids flow when rotary vibration will work. Another option could be some simple raking system, either mechanical (like a periodic windshield wiper) or a small pulse of compressed air on a timer. Maybe a combination of these fixes could be enough.

Tom Williams, sr. technical manager
Honeywell, Petersburg, Va.

Talk to the vendor

What’s the angle on your vibrating screen, including the discharge? Perhaps the angle isn’t sufficient for flow. You may want to go back to the drawing board and run a powder test to see what the optimal angle should be. Perhaps the coal for which the process was designed isn’t the same as the material going through the system now. You may need to consider some equipment changes. There are several technologies available based on your powder’s properties, perhaps a different type will work. The screen also could be blinding over with material. There also are several types of technologies available to solve this issue. I would suggest talking with the vibration screen manufacture about the issues.

Shane Foley, fibersol engineer
Archer Daniels Midland Company, Clinton, Iowa

Venting is critical

A possible cause for this build-up of material may be that you aren’t providing adequate ventilation for the “over-pressure of air” which is developed in the vibrating screen and lower hopper during normal feeding. This over-pressure of air may not allow the coal lumps to flow/move over the screen. Eventually they bind and bridge and cause the problems you are experiencing. You also should consider that the location of your dust pick-up also may be a factor in supplying stable venting to your vibrating screen and lower hopper. First, I would clean out the existing system to make ready for testing. Then, I would recommend that you re-locate the dust pick-up to the top of the vibrating screen. This should minimize the risk for coal lumps being sucked into the ducting and allow for dust collection. Next, I would recommend that you increase the suction capacity at the dust pick-up, either by increasing the velocity or cross-sectional area. This may affect your system, so please pay attention to overall system balancing. I would recommend the use of an air bleed at the dust pick-up inlet so you can adjust the flow rates so as not to over-extract material or create unnecessary vacuum in the vibratory screen. Test this system at different air bleed settings and see what happens.

Mark Smith, sales engineer,
JOA North America LLC, Charlotte, N.C.

Change the type of sifter

The screen appears to be the heart of the problem. It isn’t effective in separating dust from coal lumps. I suggest replacing it. What’s needed is something that can provide a gradual sifting instead of the wire mesh, which will easily blind. Replace the screen with a sieve tray plate with a series of holes for the dust to fall through but too small for the lumps. This will give the solids a smooth surface that may be difficult to blind. There should be some commercially available equipment that can do this type of sifting. The key point is that it must be a plate — not a screen. Sifting with a perforated plate will improve the flow of lumps to the screw conveyer.

There also is a potential for slug flow from the lock hopper valves; a surge of solids could block the feed to the vibrating equipment. I’m not sure how to do this but I know the load needs to be spread out at the feed side. Perhaps, the motion at the feed could be more rotational at the feed point to shake the pile of the lumps, flatten it, and force it to spread out. The solids could then be fed to a second vibrating screen to do the sift work.

Charles Fivelson, engineer
Carbondale, Ill.

 November's Puzzler

We operate an acid regeneration plant (ARP) for a steel mill. In the process, FeCl2 solution is oxidized to Fe2O3 and HCl in a spray roaster; the HCl is recycled to the mill and the oxide is agglomerated and pelletized for feeding back to the mill (Figure 4). Currently, operators do laboratory tests to monitor the quality of HCl (target: 18.5%). The absorber hits an azeotrope at 20.4% at which time the scrubbers become absorbers and we will be fined for an acid emission. Someone suggests monitoring the concentration of the acid in the absorber. e.g., by inferring concentration by measuring conductivity or density, or measuring directly by using an automatic titrator. Automating the absorber also is appealing because this loop requires constant attention, especially during start-up and shutdown. What should we do? Keep in mind that ARPs are water-restricted.

Figure 4. The current PFD of HCl absorption process.
Figure 4. The current PFD of HCl absorption process.

Send us your comments, suggestions or solutions for this question by October 13, 2006. We’ll include as many of them as possible in the October 2006 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

 

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