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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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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