Simulation Gets a New Dynamic

Efforts aim to boost the use of dynamic models by process and control engineers.

By Mike Spear

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In many ways, modeling has become an established tool for process engineers. Steady-state simulations are now routine for much process design work, while the more-challenging world of dynamic simulation has virtually spawned the rapidly growing market for operator training systems (OTS’) for everything from tightly integrated petrochemical complexes to offshore platforms. However, few would question that dynamic simulation could play a greater role in process engineering. The question is whether it is now poised for wider adoption by practicing process engineers.

The ability to mathematically model a process and its unit operations from first principles arguably dates back to the advent of the first computers powerful enough to do the number crunching — but those mainframe and VAX days are long gone. “Most of us in the past in the chemical industry had our own simulators, before they became commercially available,” recalls John Pendergast, senior technical leader with Dow’s Engineering and Process Sciences Laboratory in Midland, Mich. But the processing and modeling times involved then were hardly suited to dynamic simulations. “Realistically,” he says, “the ability to solve dynamic problems is only about three to four years old.”

Pendergast and his team work with the suite of simulation software packages from Aspen Technology, Cambridge, Mass., on a wide range of troubleshooting and operational issues. “There are lots of unit operations that are inherently unsteady state,” he explains, “that have to be merged into our inherently steady-state simulations — pressure swing adsorption units and batch reactors, for example.”

AspenTech’s senior product manager for both dynamic and batch simulation products, Glenn Dissinger, plots a similar timeline. “The use of dynamic simulation has grown significantly over the last six years or so,” he says, “and it has also become more closely integrated with steady-state tools.” He estimates that around one-fifth of AspenTech’s simulation customer base is using its dynamic solutions, with the popularity growing because of their “ease of use and the ability to tie dynamic simulations back into the steady-state solutions.”

The challenge

 “Ease of use,” of course, can be a somewhat relative term and it’s probably fair to say that dynamic simulation software has not always been noted for its user-friendliness — at least by the general user rather than the simulation specialist. “Even if you have a really good steady-state simulation,” says Pendergast, “porting it over to a dynamic simulation is still by no means at the level of the casual user.” Ben Weinstein, section head in Procter & Gamble’s modeling and simulation department at the company’s corporate engineering laboratory in West Chester, Ohio, makes a similar point. “The primary users of dynamic simulation from my perspective,” he says, “should be the control engineering group, but even good simulation platforms for dynamic simulation are still too difficult for control engineers to use. The big challenge is to make dynamic simulation easier to use by non-simulation people.”

If that’s so, then it’s a challenge that the software vendors are addressing. One of the growth areas picked out by AspenTech’s Dissinger is just that type of integration into the control field of the company’s dynamic simulation products: Aspen Dynamics, Aspen Custom Modeler (ACM) and the Aspen HYSYS Dynamics software it acquired with Calgary, Alta.-based Hyprotech in 2002. (The Hyprotech software was subsequently bought by Honeywell as part of a U.S. Federal Trade Commission (FTC) settlement in 2004 aimed at spurring competition in simulation, although AspenTech retains full development and user rights to HYSYS and the other Hyprotech products.)

“We’ve been working for a few years now on tying in our Aspen DMCplus model-based multivariable control package with our simulation tools,” he explains. In addition, AspenTech also recently released a new linear-state-space matrix controller, which can also link back to the rigorous non-linear models developed in ACM and Aspen Dynamics.

Another addition to the AspenTech dynamic family expected in 2006 is an extension to the Aspen Simulation Workbook introduced last year for steady-state simulations. This basically is a package that allows less-experienced users to translate their process models into the actual simulation model by quickly developing customized Excel-based front ends, rather than immersing themselves in Visual Basic coding.

Meanwhile, Honeywell in the past year has been incorporating HYSYS into its UniSim line of simulation products. The latest release in the range, UniSim Operations Suite R200, came out last December and is effectively an OTS that can also be used for control system checkout. Peter Henderson, London, Ont.-based product manager for the UniSim Operations Suite, acknowledges that the reason for Honeywell’s interest in dynamic simulation was primarily to support the OTS business <em dash>—<em dash> the type of service business that AspenTech, under the terms of the FTC settlement, is excluded from until the end of next year.

“The dynamic models in an OTS have to look reasonable at start up and under normal operating conditions,” Henderson explains. “And the path between those points has to be credible and follow a natural curve, without any mathematical non-linearities or discontinuities. Remember, an operator will see all this in his trend windows and if it’s not what he might expect, then the OTS system loses credibility.”

As well as the HYSYS offerings, the UniSim Operations Suite can also call on dynamic models from Honeywell’s own Shadow Plant and OTISS simulation engines in its Experion PKS process automation system. Shadow Plant, in turn, incorporates dynamic models produced with the gPROMS software of Process Systems Enterprise, London, U.K. Henderson notes that work with PSE started before the HYSYS acquisition and was driven by a desire to take costs out of the development of new unit-operations models. “The formulation and research time and effort is still the same, as with any good model,” he says, “but the time to turn that into an object that can be embedded into a flowsheet is very rapid with gPROMS.”

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