Lone Star Cement Plant Installs Special Wash System for Waste Tanks
Mar 1, 2000 12:00 PM
SUPPLEMENTING kiln fuel by burning waste solvents and lubricants from other industries has enabled the Lone Star Industries Inc cement plant in Greencastle, Indiana, to significantly offset its fuel costs while keeping millions of gallons of potentially hazardous materials out of area landfills.
In the early 1990s, when Indiana became one of the first states to augment the 1976 federal Resource Conservation and Recovery Act (RCRA) with stringent state requirements for tank truck cleanout, Lone Star decided to turn this potential problem into a plus by taking on that responsibility as a customer service.
With the addition of an automated tank truck cleaning system, the plant not only assures customers that their trucks will depart compliant with Indiana's "RCRA Clean" mandate, but also helps keep truck turnaround time as short as possible and avoids extra recordkeeping costs associated with unclean trucks.
Located on the outskirts of a small university town about 40 miles southwest of Indianapolis, the plant gnaws at a 1,200-acre site atop a 60-million-ton limestone reserve known as the St Genevieve formation. Rebuilt in 1969 to replace an aging 1925 facility, the plant quarries about one million tons a year, turning that into about 750,000 tons of cement.
Almost all of it goes out in bulk truckloads destined to end up in concrete. The plant also specializes in Type 3 high early strength cement favored for molded concrete products and produces masonry cement and Type 1A air-entraining formulation for freeze/thaw resistance.
Sharing this site since 1987-when the plant won EPA certification to burn waste-derived fuels-is Lone Star Alternate Fuels (LSAF), a subsidiary that locates sources of common, high-energy wastes such as leftover paints, inks and dyes, cleaning fluids, old fuel and motor oil, spent solvents, and chemical residues. Most of the suppliers are Treatment, Storage and Disposal Facilities (TSDFs) licensed to gather such wastes from local generators.
Lone Star Alternate Fuels is licensed as a TSDF and competes with other disposal technologies offering alternative ways to eliminate such wastes.
Waste Recycling "One of our main selling points," says Carlos Buckelew, LSAF general manager, "is the appealof not simply destroying the waste, but going a step further to recycle its energy content in the creation of new construction material."
Because energy content is paramount, he explains, the waste that LSAF takes in must have a certain minimum BTU value. It also can contain no more than minimum allowable substances and must be totally free of common pollutants, such as 10 heavy metals, PCBs, chlorine, pesticides, insecticides, and radioactivity. In addition, the material must be chemically compatible with other materials previously taken in, so it won't trigger unwanted chemical reactions when blended.
To assure compliance, every incoming truckload waits at the gate until samples are drawn and tested in an onsite laboratory, a process that can take 30-45 minutes. Compliant material that contains excessive water or more than 30% solids may be accepted, but at a surcharge intended to cover the cost of extra time anticipated for unloading and blending.
"Most of our sources are steady customers that contract with us to consistently deliver a specific blend of materials, which we prequalify by analysis," Buckelew says. "This sets up a chemical fingerprint making it quicker and easier to verify a customer's deliveries, or to spot any variations different enough to warrant requalifying. In this business, you don't want any surprises.
"In 1997, we burned 15 million gallons of waste-derived fuels. Typically it arrives in 5,000-gallon tank trucks, either owned or contracted by our customer companies. On a busy day, we might get as many as 20 truckloads. The site has three unloading stations, so the drivers won't waste a lot of time waiting in line.
"Truckers like that, because they know time is money. They tell us that some places keep them twice as long, and that we've got one of the quickest turnaround times in the industry."
At each station, the liquid waste is pumped out of the truck through three- or four-inch-diameter hoses and sent to nearby storage tanks, after first passing through a stilling well to trap rocks and other heavy debris, a magnetic strainer to remove ferrous materials, and finally a pair of coarse and fine grinders to puree clumps. Depending on its viscosity, the fluid can be moved through the system either by an air-operated double-diaphragm pump or a self-priming centrifugal pump.
A closed-loop vapor-balance system prevents volatile organic compounds from venting to atmosphere. Vapor pressure that builds up as the storage tank fills is redirected into the offloading truck to relieve the vacuum created as the truck empties. Any excessive pressure buildup in the loop is released only after passing through an activated charcoal filtering system that absorbs the hydrocarbons before letting the pressure escape.
The majority of incoming trucks are hauling fluids thin enough to pump and drain out naturally to RCRA clean standards in about 30 minutes, Buckelew says. Still, about 60 trucks a month come in with a residue of sludge or hard solids that sometimes amounts to half the truckload.
"We find heels right away when we draw our incoming sample, because that's done manually by sticking a pipe down through the manhole," Buckelew says.
The pipe has an open ball valve on the end, which closes upon contact with the tank bottom. This simple technique cross-sections all the various stratified materials, just as a drill core captures layered deposits in soil or glacial ice. "Sometimes the heel is so dense that the valve will close on top of it," he says. "These kinds of loads need a lot of help to be completely discharged."
RCRA Clean By Indiana's RCRA clean standard, a tank truck isn't empty if more than 0.3% of the load remains on board. A new shipping manifest would be needed before the tanker rig can leave the Lone Star facility. Paperwork aside, material left in the tank can present problems with future loads or can become a source of legal disputes over responsibility.
"With all these potential problems," Buckelew says, "we felt we'd be better off in the long run if we just make sure the trucks leave here clean to RCRA standards. That sent us looking for the best way to do it."
After studying several tank washout systems, Buckelew and process supervisor Jamie Robinson chose a robotic, fluid-jet system called the Model M7 Cleaner that was developed by Bristol Equipment Company of Yorkville, Illinois. This system uses an articulated nozzle assembly that is lowered into the tank through the manway. The nozzle directs a high-pressure stream of fluid around the interior according to a preset pattern to cut away, dissolve, and flush deposits out.
Made entirely of stainless steel to resist corrosion, the assembly includes guide bars positioned around the nozzle to protect it from damage on insertion or removal.
Low Maintenance "This system relies totally on the pressure and shape of the fluid jet to blast away heel formations," Robinson explains. "It's a very safe, low maintenance design. There's nothing there that might strike a spark, or fly apart and damage the interior.
"Another factor in our selection was the ability of the Bristol system to use whatever cleanest, thinnest solvent material is available from our storage tanks. That way we don't have to buy special washout fluids." The Bristol M7 system was installed at one of the unloading stations. A special port was added to the unit's manway hatch cover for purging the tank with nitrogen, as an extra measure of safety.
Although the Bristol design can deliver up to 250 gallons per minute at pressures up to 250 psi, LSAF operates the system at 175 to 225 psi, delivering 40 to 60 gpm. Driven by a gravity-fed 40-hp centrifugal pump, the unit provides ample jet-stream force to clean the farthest ends of the typical tank trailer.
"With this system, we can blast out a tank that's half full of very solid heel material, usually within 20 to 30 minutes," Robinson says. "The other systems we considered seemed to take noticeably longer, especially with loads having high solids content."
Mechanical controls for the Bristol unit were installed beside the truck cleaning platform stairway to allow workers to operate the system at ground level, away from contact with potentially harmful or corrosive fluid. Compressed air is converted to hydraulic force to power the system. Two hydraulic circuits power longitudinal (end-to-end) and transverse (side-to-side) nozzle movement.
Interchangeable mechanical cams that program a variety of nozzle-sweep patterns across the length and width of the tank can control nozzle movement and speed. The standard program cam is used most of the time.
"For trucks with a fairly light, loose heel," Robinson explains, "we'll start at zero (straight down to clear the tank outlet), then move the jet along the tank centerline end-to-end a few times. Then we angle the jet five or ten degrees up one side wall, run end-to-end several more times in that position, and repeat that process on the opposite sidewall.
"With heavier, tighter heels, we'll start at zero but have the jet swing back and forth in a side-to-side arc as it advances slowly toward one end of the tank, then works its way back toward the opposite end."
Nozzle Monitoring Two calibrated rods protruding from the front of the control panel move in and out in direct linkage with hydraulic cylinder movement inside the panel, serving as nozzle position indicators. These are designed for easy viewing from a distance and a wide range of viewing angles, to avoid any need to look into the tank interior to determine where the nozzle is pointing.
"The Bristol system has been very reliable," Robinson adds, "not too surprising since it looked very solidly built. It hasn't needed much maintenance, and we've never lost any turnaround time due to breakdowns."
Lone Star charges an additional flat fee for tank washout, Buckelew says, "but it's primarily to cover the cost of the nitrogen and labor. This isn't a profit center for the company.
"Our fee is considerably lower than what commercial cleaning services would charge, because they must not only make a profit on it but also cover the cost of disposal," he adds. "For us, disposal doesn't cost anything, and truck cleaning isn't our primary business.
"Being able to provide washout with unloading is actually more of a sales aid. Some of our TSDF customers say they ship their wastes here largely because they know the truck will come back RCRA clean, and they won't have the hassle of extra paper trails or the added time and expense of taking it to a commercial truck cleaner."
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