Process simulation: Your job is still safe…for now

By Wayne Labs, Contributing Editor

CAN TODAY'S simulation software systems design the product, the plant, and the control system? Maybe yes and maybe no—depending on the type of simulation. Armed with a good CAD package and mechanical design simulation software, one engineer can seemingly do it all: design a pipe, tank, or gear; define its structural and material aspects; set up a program to manufacture it, and design a plant around its manufacture. But is the story the same for process simulation?

When considering simulation, we often think of flight simulators because they’ve been around for so long. And the question follows, if we can build flight simulators, why can’t we build process simulators that can do as much as mechanical simulators, with extreme precision and 100% accuracy? While there have been major strides in process simulation software for chemical and petroleum applications, designing a process and control system for a plant is quite tricky given variables and measurement inaccuracies. Because of these challenges, the concept of a one-person design team is still a goal for the future.

Gilles Hameury, sales and marketing manager for France-based Prosim, says simulation has more than 50 years of history with time-proven models accepted by both scientific and industrial communities. These can be thermodynamic models or unit operation equations. “Given the complexity of chemical processing,” says Hameury, “100% accurate simulators are a fantasy. However, in some areas that have been extensively explored (alcohol production, for example), we can reach 95-98% accuracy, which would be used happily by any engineer.” He notes that the remaining error is due to real-life issues (product impurity, temperature or experimental measures inaccuracy) and can be ignored, adding that the simulator can be adjusted with experimental data to get close to 100% accuracy.”

The simulation packages to accomplish this degree of precision and accuracy are referred to as high-fidelity simulators (See Figure 1), and typically start at $100,000. According to Grant Stephenson, principle engineer, Honeywell Process Solutions, flight simulator software and simulation products for the nuclear power industries are perceived as absolutely necessary because the public is concerned about air safety and potential nuclear accidents. Thus, the aircraft and nuclear power industries invest in a high level of simulation fidelity. Stephenson suggests, for example, the recent Air France accident in Toronto could have been much worse if pilots hadn’t trained rigorously on simulators.

FIGURE 1: SIMULATION FOR PROCESS SAFETY

ADVERTISEMENT Controls at BP’s Saltend, UK, chemical production facility. Honeywell’s HYSYS and OTISS dynamic simulation software optimize process and train operators.

Stephenson adds, “Comparing flight and process simulators from a technical perspective, I think the ‘process’ model for a flight simulator is much better understood (mathematically and from an engineering perspective) and is easier to model.” However, today’s simulation software can handle zero flows, non-equilibrium conditions in columns, startup dynamics due to heat capacity in the walls of the process equipment, and other factors. “We do make simplifications in the flowsheet models,” Stephenson concludes, “but this is done judiciously, so as not to sacrifice training benefits.”

Low-Fi Simulation in Software
Since low-fidelity simulation software comes with most process control software packages, Dave Chappell, technology leader at Procter & Gamble, thinks control engineers would be foolish not to take advantage of it. This software, which is significantly less inexpensive than high-fidelity tools, lets control engineers verify that automation is programmed correctly. In many cases, these simulation packages are built into the control package as part of the platform. Chappell notes that P&G probably doesn’t have any two process applications that are exactly alike. Therefore, it’s reasonable to assume that modeling all these applications with high-fidelity simulation products could take a lot of time.

Todd Stauffer, PCS 7 marketing manager at Siemens, says more and more customers are considering simulation up front in their project lifecycle. In the past it was somewhat of an afterthought. Now manufacturers look to simulation as a means of reducing costs and increasing effectiveness of tasks, such as operator training, factory acceptance testing (FAT), installation and commissioning, and for testing configuration changes prior to incorporation into running processes.

Real-World Applications
Stephenson contends that dynamic process simulators can model at a much higher fidelity than exists now. This capability, however, often isn’t employed since the process industry is not governed or managed in the same way as the aircraft industry. However, there have been very serious incidents in the process industry, which might not have occurred if regulations mandating incident avoidance were stronger.

BP Chemical Production
Besides modeling processes, simulation systems ensure operational competence, safety, and plant reliability. This is the key requisite in BP’s Saltend facility (See Figure 2), located on the east coast of England, near the North Sea’s oil and gas fields. The company manufactures bulk chemicals, such as acetic acid, acetic anhydride, and acetone, at this site. Its experience with process simulation goes back to 1989. Presently, BP has three simulation systems, each equipped with Honeywell’s Icon operator consoles for use with more than 20 dynamic models, including HYSYS and OTISS, which are part of UniSim operations suite.