This Month’s Puzzler
I’ve been tasked with increasing the capacity of a screening and pressing operation at a pulp operation that uses switchback grass straw. Our chemical process relies on atmospheric cooking. The final product is for food service, so it must meet basic cGMP cleanliness requirements.
The screening and pressing operation consists of a spray screen followed by a belt press. The lights go to a hydrocyclone for cleaning before passing to our wastewater plant. The heavies collected in front of the screen go to the press.
We bought used equipment and are suffering ongoing problems in maintenance and cleaning. The belt press arrived contaminated with cockroaches, which continue to survive despite our best efforts. In addition, the belt frequently tears or spins off the rollers; solids cake to the blade and slide ramp; and water drips off the press in the wrong places. We’ve cut back our pulp feed rate and diluted the pulp extensively with city water to improve the separation at the screen. The resultant higher flow rate through the screen has forced us to shut off the outer screen valves, so we operate at about 8 psig at 200 gal/min at the inlet to the spray nozzles. The belt press dewatering pump is limited to 100 gal/min, according to a hydraulic analysis. Our reject flow to the hydrocyclone is 120 gal/min, according to the operating foreman. The belt press is designed for a maximum speed of 10 ft/min but we’re operating at 12 ft/min.
What should I include in my design report?
Take A Two-Tiered Tack
At first read, the problem presents two starkly different views. On the one hand, the situation shows scant consideration for safety and hygiene. It is possible that the decision to buy used equipment was done rather hastily. On the other hand, it offers interesting opportunities for engineering creativity. For your report, consider near-term stop-gap measures and then a long-term solution.
Short-term steps worth investigating include:
1. In view of the frequent maintenance on numerous equipment (e.g., belts, rollers and screens), consider providing spares so that replacement can be made quickly. Have maintenance look at the causes for frequent breaking of the belts or sliding of the belts off the rollers…some items to consider are alignment (rollers and belts), lubrication and supports as well as proper adjustment and tightening of the press components (given that water drips off at several unexpected locations).
2. Talk to operators and maintenance folks (people in the “trenches”); get their views as to what are the possible causes of these frequent breakdowns and what can be done in the short-run.
3. In view of the continuing cockroach problem, try disinfecting the area.
4. Because you have to use water (to the screens) at a much higher rate, it strains the water treatment system. Maybe the processing steps prior to the screening operation are inadequate.
Now, let’s consider the long-term. What is the production goal of the company for, say, the next ten years? Based on this, develop a schedule to replace equipment and controls with new equipment. To get buy-in from management, develop a detailed cost-benefit and sensitivity analysis.
GC Shah, senior advisor
Wood Group, Mustang Engineering, Houston
Do A Material Balance
If you’re already operating beyond the recommended belt speed and your de-watering pump’s rated capacity is less than the feed to the screen, you’re already struggling to keep up. I don’t trust the 200 gal/min supposedly going to the belt press: it seems like a very high reject stream flow rate. I’d start there by asking for a larger de-watering pump. It’s not unusual to get differences of opinion about flow rates, parameters and equipment depending on which layer of an organization you talk to.
Begin by developing your own material balance. Establish one between the screen feed tank and the two tanks at the screen discharge: the one feeding the belt press and the other feeding the hydrocyclone. However, don’t rely on your initial balance; use it for training. Also if the procedures are left to the operators, there will be variations. Expect to run the balance for several weeks so you can establish an average.
Find out how good the pressure reading is going into the spray nozzle and repeat with a brand-new precision gauge. If the pressure exceeds 7.5 psig, you’re at the maximum because spray nozzles discharging to atmosphere tend to atomize liquid if forced to a higher differential pressure.
As for the cleaning problem, your plant already is contaminated with cockroaches. What you should have done was to take the press apart outside of your plant and clean it thoroughly with steam and chemicals prior to letting it in the door. In addition, you should have tossed parts like belts if you suspected contamination.
Now, you’ve got the double-headed problem of cleaning while avoiding contaminating a food-grade product. I suggest using steam and caustic because they’re already part of your process and won’t cause surprises with the city wastewater plant.
Lastly, several process issues bother me. Check the performance of the hydrocyclone. If there’s no significant pressure drop, it’s being under-utilized, which means that it isn’t separating well. Cycle through it at the design rate, then pass to the next step in the process. The high use of city water will get you in trouble with the locality. Most paper mills use white water for screening; water is conserved by staging water — city water makes up the white water, white water eventually becomes black water, which is used in pulping upstream. Black water eventually goes to wastewater treatment.
Dirk Willard, consultant
At our plant, which manufactures a powder, we are suffering sudden problems with a pulsed-air cartridge dust collector. The unit, installed in 1978, is 20-ft tall and has an inside diameter of 6 ft and a 35° cone bottom. It has run flawlessly until the last month. The collector is downstream from a hammer mill in another unit. The mill is having a little trouble with wetting but otherwise runs like a Swiss watch. I’ve been asked to install an inspection platform in the collector and to resolve three reliability issues: 1) the external cartridge bags are wetted but only on one side; 2) like most dusts, ours is combustible, and collects up to 6-in. deep on some of the supports inside the collector; and 3) the wall thickness is only 10-gauge stainless steel.
What should I include in my inspection report?
Send us your comments, suggestions or solutions for this question by April 13, 2018. We’ll include as many of them as possible in the May 2018 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. 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.