01 June 2002

Kiln under pressure

By John Whitney, David Hunter, and Calvin Burnett

Field-based architecture handles regulated liquid waste

Expansion was in the air for Rineco Chemical Industries. The Haskell, Ark.-based waste management company has a plant that operates under a Part B permit issued by the Arkansas Department of Environmental Quality, pursuant to the U.S. Resource Conservation and Recovery Act of 1984. This treatment, storage, and disposal (TSD) facility handles a wide variety of regulated wastes, from solvents and paints to filter cartridges and rubber boots. Founded in 1986, Rineco employs about 350 people and processes roughly 100,000 tons of waste per year.

Some of the 68 automatic on/off valves are seen here in manifolds next to the storage tanks.

A mainstay of Rineco's business consists of adapting combustible liquid and solid wastes for use in cement kilns as a supplement to conventional coal and natural gas fuel. Finely divided solids and semisolids disperse in waste liquids-mainly spent solvents-producing a granular slurry that can pump into kilns. Other solids are shredded and packed into plastic buckets for dropping whole into kilns. Thus processed under careful control and analysis, waste sent to cement plants sees use as a waste-derived fuel. This way, the waste gets used for combustion energy, volatile pollutants face destruction, and residual ash harmlessly incorporates into the powdered cement product.

Facility face-lift

Rineco decided it needed new technology at its plant. So, the company installed a control system for its slurry blending process for kiln fuel centers on a battery of eight new 30,000-gallon storage tanks equipped with propeller agitators to keep solids in suspension.

The basic functions required of an automatic control system for this tank battery are as follows:

• Line up the valves as required for each transfer and subsequent blowdown.
• Provide alarm and safe shutdown if the possibility of improper transfer occurs, such as valve failure or manual override.
• Provide graphics and tables clearly showing each transfer path.
• Provide plain, accurate, and auditable records of every transfer.

Because of the complexity of the pipe and valve network and the supreme importance of regulatory compliance, these automation requirements are not so easy to achieve as they may seem. The primary concern was to obtain smooth approval of an operating permit that had to include every one of numerous transfers that would be needed-several even occurring simultaneously.

A key element in this system is a heavy reliance on fieldbuses: Foundation fieldbus for intelligent continuous-variable transmitters, and Actuator Sensor interface (AS-i) bus for discrete inputs and outputs. There are four Foundation fieldbus H1 segments, each of them an ordinary twisted pair providing power and two-way digital communication for as many as 16 computer-based transmitters or actuators. And there are four AS-i bus networks, each consisting of a master and as many as 31 slave modules powered through a two-wire cable, handling up to four discrete inputs and four discrete outputs per module.

The intelligent instruments using Foundation fieldbus consist of 24 pressure and temperature transmitters and five mass flowmeters. In addition, eight intelligent radar-level transmitters on the storage tanks communicate via Hart, which superimposes digital pulses in both directions on a conventional analog current signal.

The DeltaV operator and engineering stations are located in a control room that overlooks the new tank battery and drum storage area.

A compact controller with its associated power supply and I/O modules allows for instruments using other fieldbuses, such as Profibus. The system can readily accommodate other buses by corresponding I/O modules in its enclosure. In this system, Emerson's DeltaV integrates all common varieties of field communication on the same basis. It also provides integration with other control networks such as the PLCs at the blend plant.

The controller connects to two Microsoft Windows-based operator and engineering stations in the control room by an Ethernet LAN using TCP/IP. This LAN readily extends to other controllers and operator stations in other parts of the plant, eventually covering the entire facility with PlantWeb architecture.

Programming is by standard DeltaV tools that use graphic displays, drag-and-drop methods, and function sequence charts.

No turning back

Because of the system's simplicity and flexibility, it was easy to accommodate revisions in configuration details right down to a week before commissioning. That task took less than two days.

Next came six weeks of shakedown trials and training with water. During that period, Rineco was able to cure mysterious performance problems.

The company secured permits in steps, beginning with engineering design and culminating with demonstration during shakedown. One event illustrates the tone of that demonstration. A user attempted to overflow a tank while successively disabling and overriding various layers of safeguards in the system. Each time one layer was defeated, another layer shut down the transfer. Finally, after completely thwarting the system, the hard-wired interlock prevented the overflow.

One of the most valuable aspects of the system was the user friendliness of the operator interface. Rineco assigned nontechnical personnel as control room operators, letting experienced technical operators remain on the production floor where they are more valuable in the crucial work of direct supervision. These two groups in the Rineco team work together efficiently, optimizing productivity. WBJ

Behind the byline

John Whitney and David Hunter are at Rineco Chemical Industries in Haskell, Ark. Calvin Burnett is at VRC Co. in Brunswick, Tenn.