Short-Circuiting Fieldbus Installation Problems

By Tim Wilson and Jeff Marsh

Like other process industry operations, bio-fuel production plants seek state-of-the-art automation technology in order to reduce raw material costs, increase yields, comply with regulatory standards and maximize revenues. However, plant managers must ensure control systems provide reliable operation and a low cost of ownership over the life of installed assets.

Although modern, fieldbus-based process control systems offer many operational benefits, ethanol producers need effective measures to protect the fieldbus physical layer against short circuits, improper termination and other problems that can adversely affect system performance and reliability. They also need solutions enabling a quick ramp-up from installation to operation of the control system in order to improve their time to market.

Background

Industrialized nations dependent on other countries for fossil fuels have been severely affected by declining oil reserves, coupled with an uncertain worldwide economic and political outlook. As a result, the U.S. government, along with leading energy producers, is ramping up support for alternative energy sources.

Despite the rising demand for bio-fuels, the profitability equation for the ethanol industry is negatively affected by inefficiencies in production processes. An individual ethanol producer has little influence on the market price it receives for the product it ships and little control over the price of feedstock and natural gas. What ethanol producers can do – must do – is tightly control their own plant operations, and thereby produce consistently high yield, while minimizing the consumption of energy and raw materials (See Figure 1).

Figure 1. Achieving consistent profitability can be a tough challenge for bio-fuel producers. To ensure business success, ethanol plants must tightly control their production processes.

Greenfield Plant Project

In February 2007, Abengoa Bioenergy Corporation (www.abengoabioenergy.com) commissioned a greenfield ethanol production plant in Ravenna, Neb. Construction took place on the site where Abengoa built its original pilot plant in 2004. The Ravenna facility has an ethanol production capacity of 88 million gallons per year (MGY) and is one of the largest dry-mill ethanol plants in the U.S.

Abengoa Bioenergy has more than 340 MGY of total installed capacity worldwide. In addition to the Ravenna site, the company operates ethanol facilities in Colwich, Kan., York, Neb., and Portales, N. M. Its parent corporation, Abengoa Bioenergy, S.A. is one of the world's leading bio-fuels manufacturers and the largest ethanol producer in Europe.

In Abengoa's dry-mill process, corn starch is hydrolyzed into sugar and then fermented into alcohol. The major steps in the dry mill operation are milling, liquefaction, saccharification, fermentation, distillation, dehydration and denaturing (See Figure 2).

Fig. 2 Abengoa Bioenergy Corporation’s greenfield production plant in Ravenna, Neb. uses a dry-mill process to produce ethanol from corn starch.

When launching its ethanol plant project in late 2006, Abengoa Bioenergy enlisted Fru-Con (www.fru-con.com), a large industrial engineering firm, to handle plant design, project planning, procurement and construction. FeedForward, Inc. (www.feedforward.com), a control systems integrator serving the processing industries, was awarded the contract for control system design and installation.

Abengoa challenged its project team to achieve the lowest total cost of plant ownership through automation and integration from an operational and maintenance standpoint. Plant optimization required a state-of-the-art automation architecture connecting "smart" field instrumentation with a high degree of diagnostics and troubleshooting information into a distributed control system (DCS) using digital networks.

With the right automation technology, Abengoa could reduce the number of people involved in controlling and maintaining the ethanol plant, while at the same time, optimizing production by minimizing downtime and maximizing margins.

Control System Technology

After considering alternative solutions for plant enterprise automation, Abengoa decided upon Yokogawa's (www.yokogawa.com) CENTUM CS3000 DCS. This system enables true distributed control throughout the ethanol plant. It distributes control strategies to field instruments, enables flexible device networking and allows free access to the process and devices by system software.

The Yokogawa DCS takes full advantage of open, digital network standards to provide a secure control platform for the future. The system employs the Foundation fieldbus H1 protocol (www.fieldbus.org) for use with analog devices, and AS-i bus with a Profibus-DP (www.profibus.com) gateway for all motor control centers (MCCs). The major advantage of fieldbus technology, and the one most attractive to end users, is its reduction of capital and operating expenses through reduced wiring and greater information availability from field instruments. Remote configuration and asset management are two further benefits of fieldbus installations in process plants.

Foundation fieldbus is an all-digital, two-way communications system interconnecting interoperable field equipment, such as sensors, actuators and controllers, from different suppliers on a single network. The fieldbus system infrastructure reduces the amount and complexity of wiring throughout a plant (See Figure 3).

Figure 3. Foundation fieldbus is an all-digital, two-way communications system interconnecting plant field equipment.

Foundation fieldbus also transmits multiple variables, enabling a reduction in process variability, as well as device identification information. The technology allows collection and transmission of robust instrument diagnostics, thus reducing unnecessary shutdowns and improving safety and regulatory compliance.