The future of advanced process control promises more benefits and sustained value

April 21, 2003
Advanced process control (APC) has incorporated and will continue to incorporate new technologies and methodologies that allow refiners and petrochemical plant operators to achieve and sustain progressively higher levels of process operating benefits.

Advanced process control (APC) has incorporated and will continue to incorporate new technologies and methodologies that allow refiners and petrochemical plant operators to achieve and sustain progressively higher levels of process operating benefits.

Improved process modeling techniques, more flexible optimization technology, and more capable performance monitoring tools will produce the "next" generation of APC with greater economic benefits and improved reliability.

In the early days of process control, any loop with more than one input or something other than a standard proportional, integral, and derivative algorithm was considered an "advanced" loop. The focus was on regulatory performance of individual control loops.

Technology advancements included techniques to address interactions between loops, compensation for changes with differing dynamics, changes in active process constraints, operating transitions, static process nonlinearity, and multiple levels of economic optimization and integration with higher-level systems. Incorporation of all these technologies led to the multivariable constraint control and optimization technology in widespread use in current refining operations.

APC development

The most telling changes in APC since the 1980s, and likely a precursor of the future, are the things that frequently no longer exist. Unit APC justification studies, for example, no longer justify themselves because APC is accepted as an industry best practice for most major processing units; the smart money is spent on implementing projects and shortening the time to realize economic benefits.

Application computing engineers are another example: Most current APC projects are constrained by process and business expertise rather than limitations of computing hardware and custom software.

With more than 5,000 worldwide APC applications currently in service, including 2,500 in refining and petrochemicals, each generating an average benefit roughly equivalent to a 3-5% capacity increase, APC is one of the most significant technologies introduced in refining in the last 20 years.

Click here to enlarge image

The track record of APC benefits (Table 1) is well established and industry reports routinely recognize APC as an important enabling component of top performing refineries. This record is likely to continue—several new and evolving technologies show a great deal of promise for even greater benefits in future applications.

APC is a software layer in plant automation systems above the primary controls (i.e., flow loops, pressure loops, temperature and analyzer measures) that is responsible for maintaining operations within the desired limits or at desired targets, stable operations after process upsets, coordination of operating changes, and continuous drive for improved basic economics (increase feed and preferred product rates, reduce energy consumption and waste material).

Click here to enlarge image

Current APC (Table 2) technology is characterized by:

  • Commercially available technologies that generically apply to a wide variety of refinery control problems. Nearly all products use model predictive control (MPC) as the prevailing underlying technology. This is different from the early days of APC when many operating companies developed in-house technologies, which would frequently vary from application to application.
  • Proven, and cost and time-predictable project implementation methodology. APC projects typically have a payback period of 3-9 months. The capabilities of underlying instrumentation, primary controls, and information systems were historical bottlenecks, but these systems are now more frequently available and maintained for other purposes, such as monitoring and reporting. Operating companies now choose to implement APC themselves or use a competitively priced software or service provider.
  • The capacity for large controller designs. Single refinery applications can incorporate 50 or more manipulated variables (the number of loops with APC-adjusted set points) and more than 150 measurements maintained between operating limits. These larger designs provide more opportunities to coordinate compensations for disturbances or feed rate and other set point changes, and for economic trade-offs between the feed and product ends of a process unit. APC economic benefits generally scale with the number of manipulated variables included in the design.
  • Improved implementation tools, operator graphical user interfaces (GUI), and follow-up support and training. APC projects today are marked by progressively higher quality, improved supporting tools, and well-defined overall packaging.

APC originated in the late 1970s and early 1980s in the process technology groups of major refiners and petrochemical companies, which had the economic focus, technical skill, and wealth of potential applications to drive its development.

Many other industries benefited from these developments because the same APC.technology was applied to bulk chemicals, specialty chemicals, plastics, polymers, cryogenic air separation, pulp and paper, solids and drying, building products, life sciences, and several other industries.

Whereas these industries benefited from early technology developments, refiners and petrochemical companies will likely benefit from a wide user base, driving a broad set of future APC technology developments.

Evolving technologies for APC are poised for greater economic value and operations benefits through:

  • Sustained value (high application up-time and reliable benefits) and continued improvements in economic performance driven by performance monitoring technology. This is currently one of the most active areas of academic control research and product development by commercial leaders and new APC suppliers.
  • Reliability driven by robust control and optimization technology. This is a critical requirement for sustaining benefits and integrating APC with higher-level planning, scheduling, and optimization systems included in enterprise resource planning systems.
  • Improved process modeling driven by state-estimation techniques, nonlinear modeling, and adaptive control technology. These improvements will directly translate to improvements in controller dynamic performance and economic optimization.
  • Console operator tools that improve overall experience and work processes associated with APC.
  • Engineering tools that will lower the initial cost and shorten the time to benefits for new APC applications.
  • The impact of other related technologies, such as multivariate statistics, alarm management, data link standards, and wireless communications.

Sustaining value

Click here to enlarge image

Perhaps the most active areas of APC technology progression are developments related to performance monitoring and sustaining the value from APC applications (Fig. 1).

Process equipment modifications, changes in operating strategy, feed rate and quality changes, debottlenecking, instrumentation degradation, and fouling can contribute to deterioration in regulatory and economic performance of an APC system.

Disturbances due to changes in other units, changes in operating mode or point, new integration with higher-level systems, and more frequent changes in processing rates can contribute to changes in APC performance requirements.

APC applications are a critical component of enterprise manufacturing systems. They typically represent the point at which a higher-level strategy and plan adjusts the process and is adapted for real world disturbances and modeling inaccuracies. APC application performance can degrade with process changes, and performance requirements continually increase.

Because typical economic benefits of an APC application range from 3% to 5% of operating cost or capacity, protecting even a 1% decrease in benefits justifies performance monitoring and a focus on sustained value. Several developing technologies promise to eliminate susceptibility to poor performance and produce greater stream of APC benefits:

  • Performance parameters to isolate the source of performance degradation and produce a clear path to continued improvement. These parameters incorporate developments in closed-loop identification technology and multivariate statistics.
  • Tools to monitor and diagnose performance of the underlying control and instrumentation system. Impacting technologies include multivariate statistical methods for fault detection, discrete optimization, and knowledge-based methods of causal analysis.
  • Process-state definition and classification. Technologies that automatically define the process conditions and expected performance level are critical to identifying opportunities for improved performance.
  • Automated testing and identification, and adaptive control. MPC controller performance is directly related to the accuracy of the process model. These technologies increase accuracy by producing optimally informative process data and appropriately adjusting the model.
  • Knowledge-based analysis tools that identify and prioritize opportunities for performance improvement and suggest changes such as adding a feed-forward variable, retuning for a new operating condition, ensuring proper limit values, filtering a noisy signal, or remodeling a particular portion of the controller.

These technologies define APC performance, diagnose and solve problems, and identify opportunities for continued economic and operational improvement. Future APC applications will include technology to sustain the high level of benefits, more capable on-line products to make improvements automatically, and new work processes around the tools that protect important APC investments.

Reliability

In the early days of APC, practitioners and users strived to design and develop applications with a 70-80% in-service time. APC controllers are now expected to regulate and optimize within a wider range of operating conditions and maintain service factors of nearly 100%.

In addition, new planning tools and integrated business processes transform changes in market conditions quickly into process operations. Operating set points are adjusted more frequently, prevailing operating strategies changed, and in general, APC applications are expected to operate with greater flexibility and reliability.

Improvements in underlying optimization and control technology will increase future APC application reliability:

  • Robust control technologies promise to maintain controller dynamic performance despite changes in the inherent process dynamic response. These techniques frequently consider that the process responds over a range of possible values, rather than to a specific point. APC applications will therefore be less susceptible to modeling error, inconsistent measurements, and inconsequential changes in targets.
  • Robust optimization technology enhances performance by targeting the most reliable operating point within the true resolution of the process and prevailing economics. APC applications improve by producing the most operationally acceptable solutions while still nearly achieving the desired level of economic optimization. Actual cost data, rather than values adjusted to achieve a designed response, optimize the APC objectives.

These technology advances are easy to implement; economic optimization and dynamic regulatory performance are enhanced with little or no user involvement. Increased reliability and an inherent tolerance for changes in operating conditions and strategy will characterize future APC applications.

Improved core technology

APC users quickly determine that pursuing substantial economic benefits means significantly changing process operations, in terms of which manipulated variables are moved, the size of moves, and operating the process closer to true operating constraints. This pushes the capabilities of the process and underlying technology.

Click here to enlarge image

Improvements in the underlying APC technology will lead to new APC designs (Table 4), improved performance, and a wider scope of applications:

  • New optimization algorithms will incorporate discrete (integer) variables such as pumps, cooling fans, discrete valves, stream selection, and operating modes into the APC design. This will integrate the continuous, dynamic process operations with discrete operations that significantly impact dynamic and economic performance.
  • State-space modeling structures will provide more accurate process predictions through multivariable reconciliation of controller feedback, disturbance models that better describe the full impact of process upsets, and more effective use of secondary process measurements. State-space controller structures will provide more accurately calculated control moves and less susceptibility to wide ranging (fast and slow) process dynamics. This will result in tighter control and more capable controller designs.
  • Continued developments in identification and, more generally, model development technology. New technologies can extract more information from existing process data, take advantage of model structure and secondary measurements, and include automated conditioning of data to remove noise, calibration errors, and other anomalies. Tools that provide translation of relevant information from other model sources, offline for design and online for control, are in development.

Nonlinear control technologies are finding successful application generally outside the refining industry, and particularly in polymers. These technologies, along with nonlinear state-space technologies, may be included in refining applications in the longer term.

Primary benefits include the capability to handle severe operating mode changes and stability for highly non-linear measurements like distillation properties.

Operator tools

Recent developments in web (plant intranet) interfaces and performance monitoring tools have made detailed APC application information widely available to console operators and supporting process engineers. Wireless networking promises to make more information available to operators quickly.

Console operators will focus less on the process as a series of measurements and alarms, and more in terms of current state, operating strategy, and tactics. Because APC provides a significant portion of the strategy and tactics, it is essential to have tools that describe plant operations in these terms.

Click here to enlarge image

Table 5 shows the current strategy and tactics of an APC application in relevant process terms. The optimization objectives are sorted by priority and details the basic optimization objective, manipulated variable, and constraining control variables. This will change as process disturbances enter the process, operating limits are adjusted, or control objectives change.

Click here to enlarge image

Fig. 3 shows a tool that details the impact of process manipulated variable and disturbance variable changes (rows) on observed changes in process measurements (columns).

This focus on strategy and tactics is critical; integration with higher-level enterprise resource planning systems will potentially allow automated strategy changes, which are difficult for operators to track by looking at measurements.

The trend of operators worrying less about change-to-change variations in the process, and more about overall performance and the source of upsets, will also continue. APC applications will include new variable types and encompass a larger scope, again moving the operator's focus from the logistics of running the plant to monitoring overall performance.

Support engineers will also need new tools to quickly understand and change the tactics, rather than the tuning, of APC applications. APC tools with more embedded and less exposed technology are needed to support these new work processes. Some technology will reside in the underlying APC and some in the user interface, which will provide a more effective experience when developing and interacting with the APC application.

New modeling technologies will also change the way operators and engineers track process disturbances. A side benefit of state-space technology, for example, is a clearer definition of the exact disturbances entering the process (as described in a multivariable context). This provides a clearer basis for prioritizing performance improvement opportunities related to process upsets, and integrating initiatives such as Six Sigma into APC sustained value work processes.

APC future applications

The future does not include the degradation of APC benefits, applications turned off due to changes in strategy or operating conditions, isolated APC applications, and operators monitoring the move-to-move controller performance. The future does include more benefits from better technology.

The future of APC in refining is continued growth in the number of applications, tools and technology to sustain benefits, increased capability of the underlying technology, integration with higher-level applications, and better tools for operators and engineers to improve performance and to shorten the time to realize benefits.

These improvements, and the progression of related APC work processes, will produce greater initial economic benefits, those benefits sustained during the lifetime of the controller, and more reliable operation as APC becomes the key to future integration strategies.

Click here to enlarge image

The author
William M. Canney ([email protected]) is a principal technologist with Aspen Technology Inc., Houston, responsible for APC technology development. He has more than 20 years' experience with APC and MPC technologies, both as a practitioner and technology developer, having also worked for Union Carbide Corp., Praxair Inc., and Dynamic Matrix Control Corp. He holds a BS in chemical engineering from the University of Wisconsin-Madison.