The BNL polyethylene encapsulation process centers on the use of a versatile, industry tested, single-screw plastics extruder. Dry or pre-treated waste and polyethylene binder are continuously fed to the extruder by individual dynamic feeders. Distributive mixing within the extruder produces a homogeneous molten mixture that is extruded directly from the die into a waste container. The molten mixture is allowed to cool forming a solid monolithic waste form.

The Full-Scale Demonstration Facility at BNL features a fully integrated encapsulation process including waste pre-treatment, material conveying, precise metering of feed materials, extrusion processing, on-line QA monitoring and process control. This facility was developed following bench-scale formulation and testing of the polyethylene encapsulation process. The BNL full-scale encapsulation system is a scale-up of the bench-scale encapsulation process and was designed to match production capacity at a typical DOE treatment facility. The schematic shown below of the polyethylene encapsulation process showing the primary system components. The waste pre-treatment system includes a vacuum dryer which discharges directly (gravity feeds) through a grinder into a storage drum. The pre-treated waste and low-density polyethylene (LDPE) binder are then conveyed (via material transfer system) to the feeder storage hoppers. The waste and binder materials are accurately metered to the extruder feed throat where they are extruded directly into a waste receptacle. The on-line monitor provides a real-time analysis of the waste loading in the extrudate as it exits the extruder die. A low-profile floor scale under the waste receptacle is used in conjunction with a computer for the process control loop.

Pre-treatment of aqueous wastes is required in order to make them amenable to extrusion with polyethylene. The polyethylene encapsulat ion process operates between 120 - 150 °C, above the volatilization temperature of water. For effective encapsulation processing, all residual moisture must be removed from the waste prior to encapsulation. In addition, the waste must be a granular or powdered solid within a pre-determined particle size range for successful feed/metering and mixing within the extruder.

Rotary Vacuum Dryer
A Liquid Volume Reduction System (RVR-200) was supplied under contract by Vectra Technologies, Inc., Columbia, SC. The dryer is a skid mounted system with separate modules for the blender/dryer, steam generator, condensate recovery and chiller. Dryer capacity is rated at 757 liters/day (200 gal/day) equating to 408 kg (900 lbs.) dry salt/day for the nitrate salt surrogate containing 35 wt.% solids. The dryer was specially fabricated for the BNL Full-Scale Test Facility and incorporates design features to allow flexibility in testing various wastes. The Vectra RVR-200 system uses a 425 liter (15 ft3) blender/dryer vessel constructed of type 304 stainless steel. The mixing blades are also constructed of type 304 S/S and are arranged in a double helix with opposing pitch design. The ribbon blades are bolted to the main shaft to allow modification of the mixing blade design, e.g., replacement of the ribbon blades with plow blades. The shaft is driven by a 25 hp motor through a parallel gear reducer that drives the agitator/scraper assembly at a rate of 20 rpm. The blender/dryer vessel is equipped with four, 5 cm (2 in.) flanged ports to allow for the addition of optional high speed choppers. For certain wastes, high speed choppers may be useful in preventing clumping of the material during the "dough" phase of the drying cycle. Heat is provided by two electric steam generators that pressurize the blender/dryer steam jacket to approximately 10-15 psig. The liquid waste is heated and steam vapor is drawn from the dryer through a demister/filter assembly to a condensate system cooled by a 15 ton chiller unit. The blender/dryer discharges through a bottom 15.2 cm (6 in.) diameter, pneumatically operated ball valve, into a variable speed grinder to reduce particle size, if required. The final product is emptied to a storage drum maintained under negative pressure to prevent powder dispersion. Photographs of the RVR-200 unit installed at BNL are shown, including the blender/dryer skid, the condensate skid and the chiller skid.

Size Reduction
Comminution was used for size reduction and was integrated into the waste pre-treatment system. Dried salt is gravity fed from the blender/dryer discharge valve directly through the grinder to the waste collection drum. A comminutor consists of multiple, reversible, rotating knife blades which impact the material until the particle size has been sufficiently reduced to fall through a semi-circular sieve screen. Figure shown to the right is a photograph showing the comminutor multiple knife blades. Particle size is adjusted by replacing the sieve screen. A 3.2 mm (1/8 in.) sieve screen was used to process the RFP surrogate. Comminution was successful in producing a particle size distribution that was amenable to extrusion.

Pneumatic vacuum conveyors, manufactured by Vac-U-Max, Belleville, NJ, were used to transfer the pre-treated powdered salts and the LDPE pellets to extension hoppers on the material feeders. Each feeder was fitted with a 142 liter (5 ft3) hopper extension that increased the feeder storage capacity to 167 liters (5.9 ft3). The main components of the vacuum conveying systems are control panels, suction wands, receiving tanks, low-pressure valves, and rotary positive displacement vacuum pumps. The waste material transfer system uses 64 mm (2.5 in.) suction lines in conjunction with a 7.5 hp vacuum pump. The LDPE pellet transfer system uses 76 mm (3 in.) suction lines with a 10 hp vacuum pump.

The control panel contains a series of binary timing switches that control the discharge and transfer cycles of the overall material transfer process. The control panel receives a signal (contact closure switch) from the feeder computer controller when a preset "heel" or empty point is reached in the feeder storage hopper. The Vac-U-Max control panel then engages the vacuum pumps, moves the low-pressure valve to draw atmospheric air, opens a bottom slide/discharge valve and then waits for the discharge timer to elapse. This first discharge step ensures that no material is in the receiving tank before switching to the transfer cycle. Following the discharge cycle, a brief pulse of compressed air cleans the pre-filter in the receiving tank, the slide-valve closes, and the low-pressure valve switches positions to pull a vacuum on the receiving tank and the suction wand. The system cycles in this manner until a "full" signal (full hopper) is received from the feeder computer controller, based on a weight signal from the loss-in-weight feeder scale.

Material feed was accomplished with two dry material feeders, AccuRate Model 610, Whitewater, WI, converted to a loss-in-weight (LIW) system, Merrick Model 510, Lynn Haven, FL. This system provided accurate pre-determined mix ratios of waste and binder to the extruder. The LIW control system consists of three computer controllers arranged in a feedback loop: a master controller and an individual ("slave") controller for each feeder. The master controller performs a number of functions including (1) accepting a manual process setting or variable signal from the process control computer as the total feed rate, (2) setting the waste/binder ratio according to an entered recipe, and (3) controlling the individual feeder modules to maintain the feed rate and the waste/binder ratio. A "pacing" feature maintains the waste/binder ratio by decreasing the total feed rate if an underfed condition is sensed by either individual feeder module. The individual feeder control modules function to (1) accept a manual feed rate or the master total feed rate based on the waste/binder ratio from the master controller, (2) adjust the speed of the feeder to maintain ñ 1% of desired delivery, and (3) signal the material transfer system when a "heel" or "full" point is reached in the hopper level.

For the Technology Demonstration, a 11.4 cm (4.5 in.) single-screw extruder, Davis-Standard, Pawcatuck, CT, with an output capacity of 900 kg/hr (2,000 lb/hr) was selected. The capacity of this extruder when fitted with a 6.4 cm (2.5 in.) diameter die, as during the demonstration, was measured to be approximately 450 kg polyethylene/hr (1000 lb/hr). The extruder is equipped with five barrel zones and two die zones with thermocouple controllers which provide gradual heating of the waste/binder melt. The barrel heaters are cast aluminum electric "clamshell" type heaters. A cooling loop consists of distilled water circulation, a flow-through water-cooled heat exchanger, and individual zone flow indicators. A solid-state dual probe anticipatory temperature control system holds the barrel temperatures to ±1°F. The screw is driven with a 150 hp drive. The extruder is also equipped with a two-stage vented screw with feed transition and metering sections in the first stage, and vent and metering sections in the second stage. The vent is used to pull off residual volatiles (e.g. small amounts of moisture in the waste) by a closed loop, oil-cooled vacuum pump. The production-scale extruder is shown to the left.

On-Line Infrared Monitoring

An on-line monitor developed at Ames Laboratory was used to provide real-time waste composition data for the waste/binder mixture as it was extruded. The monitor is based on a technique known as transient infrared spectroscopy (TIRS). The monitor is installed at the end of the extruder and is operated by inducing a small temperature differential on the surface of the extruded melt, reading the infrared spectra, then providing computerized analysis to convert the spectra to a waste composition. The instrument provides real-time data on the actual waste loading of encapsulated waste exiting the extruder by calibrating the monitor with spectra for known waste loadings. The TIRS system enables the operator to continuously monitor the current product waste loading and to check for any variations in the waste/binder ratio for quality assurance and quality control purposes. Click here for a photograph of the TIRS monitor in position at the extruder die.

An open-loop, integrated process control and data acquisition system coordinated material feeding and extrusion processing. Process control is accomplished using a customized computer program written with Windows-based software (LabVIEW for Windows, National Instruments, Austin, TX) installed on a standard IBM compatible PC. LabVIEW operates through graphical programming as opposed to code-based programming languages, such as FORTRAN and C. In LabVIEW, graphical icons are wired together to mimic subroutines and operations loops. A multifunction input/output data acquisition (DAQ) board was installed inside the computer capable of analog input (A/D conversion), buffered data acquisition (high-speed A/D conversion), analog output (D/A conversion), digital input/output, and counter/timer operations. Connected to the PC was an external SCXI (Signal Conditioning eXtension for Instrumentation) chassis used to condition input and output signals. During data acquisition, signal conditioning is often required. For example, conditioning for thermocouple signals includes amplification, linearization, and cold-junction compensation.

For the polyethylene process, the process control computer monitors the extruder output rate by weighing the quantity of material being extruded. A low-profile weigh scale located under the drum being filled sends a signal to the process computer via an RS-232 serial port. Within LabVIEW this signal is integrated to an output rate that, in turn, is sent through the SCXI chassis as a variable analog signal (0-10 V) to the Loss-in-Weight Master Controller as the updated total feed rate. This method allows the feed rate to match the extruder output rate. The process control computer also monitors the extent of drum filling, signaling the operator when the drum is full.

 

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