Urea is produced in prills, granules, flakes, pellets, crystals and solutions, but it is traditionally marketed as prills or granules. The choice of the commercially preferred form is normally associated with its application, solubility, crushing strength and free-flowing behaviour. Pneumatic conveying and storage of urea must take into consideration the abrasive, hygroscopic and heat-sensitive characteristics of this compound.
The author: Stefan Kalt Global Business Development Manager Plastic Compounding and Extrusion, K-Tron
Handling and storage of urea require special attention to several product related properties and external factors. Most common problems are associated with moisture pick-up, caking, dustiness and particle segregation. The factors that particularly affect the moisture absorption-penetration characteristics of urea are its chemical composition, particle porosity, particle surface area and degree of crystallinity. Caking is the formation of salt bridges or adhesive points of contact between the particles. The majority of solid urea product is bulk shipped in trucks, enclosed railroad cars or barges but approximately 10 % is bagged. The flow diagram shows the rail unload and in-plant transfer operations in a melamine and urea resin facility for the production of break-resistant melamine tableware. Urea is received in railcars and unloaded into one of two storage tanks. The material is conveyed on demand by pressure to one of two scale hoppers, which supply six reactors.
The system consists of a dual blower rail unload system with a filter receiver group capable of handling up to 9000 kg/h or more. The vac-uum side of the system moves the material from the railcar to one of the filter receivers, which effectively separates it from the conveying air. Mounted below the receiver is a K-Tron heavy-duty (HD) Aerolock rotary valve that meters the material into the pressure side of the system for final delivery to storage. The Aerolock rotary valve is made of stainless steel and includes relieved tips to reduce material smearing.
Automatic bin vents are placed on top of each storage tank to trap the airborne particles of material inside while allowing the conveying air to escape from the tank as it is filled. The trapped particles are removed from the unit’s filter bags by timed, compressed air pulses. The conveyed material remains in the storage tank and clean air exhausts through the bin vent into the atmosphere. After-coolers are normally installed at the discharge of the pressure blowers to keep the temperature of the conveying air from reaching excessive levels and avoid changing the material’s characteristics.
Due to urea’s hygroscopic nature, the conveying line from the railcar to the filter receiver is insulated in order to eliminate condensation in the line as a result of ambient temperature changes. Desiccant bed dryers (DBD) provide a blanket of dry air on the material while it remains in storage. Diverter valves allow a single conveying line to be used to produce the dry air blanket and deliver material. When the railcar is unloaded, the valve diverts to the line conveying the material. Following the unloading cycle, it automatically diverts back to the supply of dry air. Desiccant bed dryers permit fast, efficient drying when urea is stored.
The second phase of the system is the pressure conveying part that allows the customer to transfer urea from either of the storage tanks to one of two scale hoppers located above the reactors. The system also enables material to be conveyed from the first storage tank to the second. Bin unloaders are positioned at the discharge of each storage tank to agitate the urea and ensure an even flow to the Aerolock feeding the pressure system. The Aeropass valves arranged on top of each scale hopper mean both hoppers can be fed with a common material line. When one destination requests material, the Aeropass valve diverts to the 90° position. The material and air are directed into the hopper, where they separate; the material stays in the hopper while the air passes back to the source tank. When the scale hopper’s preset weight is reached, the valve returns to the straight-through position and the feeding Aerolock stops, permitting the material and air to purge to the source tank. This closed-loop design requires less headroom and eliminates the need to provide separate filtering on each scale hopper, resulting in a cleaner and less expensive installation. When the reactor asks for material, the slide gate on the scale“s discharge opens. Multiple ‚A‘ valves situated beneath the gate direct the batch to the reactor that requested the material. To prevent the reactor’s fumes and moisture from reaching the scale hopper and contaminating this material, additional slide gates are located directly above each reactor.
A PLC control panel, supplied by K-Tron, facilitates automatic operation of both the rail unload and the in-plant scaling systems. Using the touch screen monitor, the operator has control of both systems including starting and stopping, source and destination selection and the scale setpoint inputs. The dynamic display screen also provides detailed flow graphics showing the status of each system. By scrolling through the various screens, the operator can monitor alarm conditions, verify which motors are on or off and check the position of each diverter valve and slide gate. To optimise the company’s material management, the PLC panel also includes an interface to each scale instrument, to enable accumulated totals to be displayed on the monitor. The PLC is also expandable to meet the customer’s future system requirements.
Outdoor area, Booth A27