Which Advanced Process Control (APC) Strategy and Architecture to Use for Chemical Plants

Chemical plants operate at the intersection of complex multivariable interactions, stringent product specifications, and economic optimization demands. One critical decision often determines whether facilities achieve exceptional profitability or settle for mediocre performance: choosing between DCS/PLC-based Model Predictive Control (MPC) and commercial MPC software solutions.

Both options use advanced multivariable optimization, but their cost, complexity, transparency, and lifecycle burden are dramatically different. Understanding when each approach makes sense prevents costly implementation mistakes and ensures that MPC investments deliver sustainable value in real chemical plant operation.

The MPC Promise

Model Predictive Control operates fundamentally differently from traditional PID approaches. Instead of reacting to control errors, MPC:

• Predicts future process behavior using dynamic mathematical models
• Optimizes control moves over prediction horizons
• Explicitly considers constraints
• Continuously updates based on new measurements

For complex processes, such as integrated oil and gas plants, MPC can unlock economic value unreachable with standard regulatory control alone.

The Hidden Costs of Commercial MPC

Commercial MPC platforms (Aspen Tech DMC, Honeywell Profit Suite, Yokogawa PACE, and others) run on separate computers connected over the OPC communication protocol to the customer DCS or a PLC. While powerful, they introduce substantial hidden costs:

• High project costs: typically, $200,000–$800,000+, requiring approximately 5–10 months of project execution
• Additional IT infrastructure: Separate expensive servers, OPC links, MPC software license, cybersecurity hardening, watchdogs, OS patching, backups
• Ongoing licensing and support: extra charged continuous vendor dependence and recurring annual costs and maintenance fees
• Specialized expertise: Often becomes a "black box" accessible only to specialists, and requires specialized training for operators and plant engineers for MPC operation and maintenance

One industry reality: many MPCs are turned off after a few years, due to lack of knowledge and inability to adapt them to new process, equipment, and economic conditions, which by their nature are constantly changing.

DCS/PLC-Based MPC Approach

DCS/PLC-based MPC is Advanced Process Control (APC) completely embedded inside the customer DCS/PLC — No Commercial Software or Annual Fee Baggage.

It implements genuine MPC elements directly inside your existing DCS or PLC system — without standalone commercial software platforms.

This approach:

• Easy to use, since its functionalities are already known to all operators and process control engineers — no need for new complex training
• Easy for maintenance since it runs natively in the customer’s well known DCS/PLC — no need for continuous vendor support
• Modular for operation and frequently changing plant conditions — allowing certain APC logics to be easily turned off, while keeping others running and providing monetary benefits
• Simple for design — eliminates the usage of any commercial APC software, server, licensing lock-in and OPC setup, cybersecurity hardening, watchdogs, OS patching, backups, etc.
• Faster in operation and successful in fighting unmeasured plant upsets and unmeasured disturbances — runs at much faster DCS/PLC scan rate and has powerful proportional kick and derivative action, an excellent antibiotic for disturbance rejection.
• Very easy to re-optimize and update (adapt to new process conditions) — it uses stepless closed-loop system identification and multi-objective optimization technology.

The result: cost-effective multivariable control that often matches or surpasses commercial MPC performance, especially when:

• The process is behaving as a serial process (not entirely multivariable and heavily interacting), which is the case in 75% of worldwide processes.
• Plant is suffering from constant capacity or condition changes or is affected by unmeasured disturbances.
• Production is so sensitive that it does not allow intrusive and time-consuming step-testing for regular APC improvement and maintenance.
• Budget, timeline and manpower are constrained.

Across dozens of industrial sites, DCS/PLC-based MPC often delivers comparable — or superior — results compared to commercial MPC, at one-third to one-half of the total cost.

Quick Decision Criteria

Making the Right MPC Choice

The process control industry has long promoted commercial MPC as the automatic "gold standard." Reality is more nuanced:

• For most chemical plants (75% of cases): DCS/PLC-based MPC delivers real optimization, lower cost, less complexity, and complete transparency
• For the most heavily interacting complex sites (25% of cases): Commercial MPC can be justified and valuable — if implemented correctly

The difference between an overbuilt commercial solution and a right-sized DCS/PLC-based MPC application can reach millions of dollars in operational cost over the system's lifetime.

PiControl Solutions provides the assessment framework and expertise to help you choose the right MPC architecture — whether that's DCS/PLC-based MPC or commercial MPC software. The goal: stable, optimized, profitable operation with a control solution your team can truly own.