Achieving operational excellence using APC

In the chemical industry, advanced process control (APC) is the formula for reaching higher levels of operational excellence. See 10 APC opportunities, including 3%-5% capacities gain and 3%-15% energy savings.

By Tushar Singh, Kate Kulik December 2, 2016

Operational excellence is a business necessity to achieve and maintain a strong market position within the chemical industry. A way to gain a higher level of operational excellence maturity is with a well-planned advanced process control (APC) implementation program. While the goal of APC implementation in the chemical industry is simple: to maximize margins while meeting customer expectations, there are a few factors chemical operators must consider to ensure a successful implementation. 

Understanding the benefits of APC

When APC was originally introduced in the early to mid-1980s, it was designed for refineries and petrochemical sites. Since then, this technology has evolved. APC is a process control and optimization technology that takes into account the multivariable interactive nature of process units to reduce variability and drive the process to an optimum on a minute-by-minute basis. This is done by manipulating variables, such as feed flow, temperature settings, pressure settings, and reflux flows, which are normally changed by operators who run the unit.

Due to the multivariable nature of process interactions and variability inherent in process units, operations cannot fully optimize the process manually, as this will result in key variables moving out of the desired range of operation from time to time. To be on the conservative side, operations are forced to move the process out of optimum and into a more comfortable operating range (represented by "A" in Figure 1). However, this often results in higher costs and/or lower production rates. There is more energy usage, lower yields, and lower feed rates compared to operating with APC.

For decades, APC has proven to reduce the variability in the process (represented by "B" in Figure 1) by actively controlling key process parameters on a minute-by-minute basis. The controlled variables are stabilized, and the standard deviation is reduced. When the key variables are stabilized, the optimizing nature of APC is used to move the process to a more optimal point (represented by "C" in Figure 1). This results in consistently operating the unit close to the maximum possible profit day in and day out, safely and reliably. 

Identifying APC opportunities

The first step when implementing an APC program is to understand what process units or part of the units are good candidates for APC. To be able to quantify benefits and improvement opportunity, it is necessary to first understand the economics of the unit and the production process. It is also necessary to consider the unit by itself, or in many cases, its role within the bigger production value chain.

If organizations have business or process key performance indicators (KPIs), they can serve as a good starting point for analyzing the APC opportunities. However, there are other ways potential APC benefits can be calculated.

The following 10 criteria can be used to quantify the benefits associated with an APC project.

1. Production increase: Typically, APC projects have proven to increase capacity by 3% to 5%. By reducing the variability in the process and operating closer to limits, APC debottlenecks the process, or part of the process, allowing higher production rates. For this to happen, however, an understanding of how the intermediate products affect downstream units and why is required.

2. Energy savings: Energy savings from APC implementation have been reported to be in the range of 3% to 15% depending on the process and current operations. Most of the time, utilities management, especially steam, is a complex control issue that spans across the site and at times also affects the electric grid. APC applications can be designed to manage utility systems effectively by matching steam production to the site’s demand. Benefits come from reducing pressure let-downs and reducing or eliminating vented steam onsite. Many companies report a 60% to 90% reduction in vented steam using APC. After the steam header pressures are stabilized, APC also can be used to optimize energy production. For example, boiler loads, gas turbines, or other sources of energy can be adjusted to maximize the efficiency of the overall system while ensuring stable header pressures, day in and day out.

3. Quality improvements: It’s important to reduce variability in the final product quality. Some products—for example, certain types of polymers and specialty chemicals—are sold at a value that depends on the quality variability of the batch produced. Many companies report a reduction in standard deviation of product qualities of up to 50%.

4. Yield improvement: Many organizations experience a yield improvement of 1% to 2% through APC deployments. Typically, this is achieved by optimizing the reactor part of the process and/or the separation portion. In almost all cases, the reaction is highly exothermic or endothermic, meaning good reactor temperature control is key. In addition, good control and optimization of the ratio of feed components to catalyst are very beneficial. Well-controlled reactors typically see an increased yield of 0.5% per pass at the same throughput rates, while maintaining safe operating temperatures. In some cases, it has been proven that a well-controlled polymer reactor online time is increased by up to 5% before a shutdown is required to clean out the reactor, resulting in improved yields and throughput.

For the nonreaction part of the process, such as distillation columns, maintaining the specifications on the final product can increase the yield of the desired product by increasing impurities up to the contracted specifications. Higher amounts of lower value products (impurities) in the final product are sold as higher value product, while maintaining the specifications and customer expectations.

5. Optimizing the polymer production wheel: In polymer production, the production wheel usually is not optimized to market needs, as difficult transitions may be rejected by operations. Without APC, grade transitions take longer and result in low-value products produced during these transitions. It’s common to see a 20% to 50% reduction in grade transition time. Through APC, it is possible to "bust the production wheel" and produce grades when they are in demand, while minimizing the time when the low-value transition products are produced. Companies using APC in conjunction with supply chain software can exploit the new capabilities to further optimize the production wheel (see Figure 2).

6. Recycle ratios: Units that have recycles are good qualifiers for APC. The addition of the fresh feed components depends on the quality of recycle. Without APC, a fixed ratio of fresh feed to recycle is maintained. This does not consider the product quality, downstream equipment operation, or the unit constraints. The multivariable nature of APC allows for optimizing the fresh feed-to-recycle ratio to maximize throughput and/or maintain quality at specifications.

7. Emissions control: APC implementation also can help control environmental constraints. By using APC to model and actively control the furnaces and boilers on a steam utility plant through APC, it is possible to operate closer to the emissions limits without violating them. It also helps decrease energy usage and minimize costs to meet NOX and SOX emissions constraints.

8. Exploiting ambient temperature effects: APC has proven to exploit the benefits associated with ambient conditions. Ambient temperature has an effect on compressor performance, condensation capacity, gas turbine operation, cooling water temperature, refrigeration capability, gas volumes, and many more process-related conditions. Diurnal effects, humidity, or even cloud cover have an effect on cooling water temperature, which can result in changes in compressor throughput limits. Operators cannot be expected to exploit the benefits associated with ambient temperature changes. This would mean anticipating the ambient temperature effects on the process and adjusting the process parameters only to reverse all changes as the sun rises in the morning. Units that are affected by ambient conditions can benefit from APC applications as they can anticipate these changes and adjust process parameters accordingly on a minute-by-minute basis.

9. Dynamic opportunities: Without APC, disturbances arising from upstream or downstream will affect process units and reduce margins. For example, in temporary situations where feed quality is decreasing by 1% to 5%, operators will react to keep the product on spec, but most likely not in an optimal way. When things are more stable, the unit may not be optimized because the situation is temporary. This is at best a means of producing on-spec product but at a very large giveaway. At worst, it would mean decreasing feed to cope with the situation. With APC technology, units will constantly react to the disturbance optimally.

10. Site-wide optimization opportunities: Operating units with APC presents a larger scope of optimization opportunity for the site. Optimizing a part of the process as a standalone would mean lost opportunity compared to optimizing multi-units together. For example, pushing a reactor to maximum throughput might not make sense if the bottleneck of the unit is the off-gas stripper. If this happens, light materials that should be removed are pushed into either flare or off-gas where they are lost. At some point, this may mean reduced margins. These interactions and constraints are considered in the design of APC systems and can lead to significant benefits for the overall site. 

APC deployment and project lifecycle

After identifying the business case for an APC project, the next step is to start building and implementing the APC controller. Traditionally, APC projects have been long and expensive with many defined steps needed for successful APC commissioning. For example, organizations would be required to go through countless phases of step testing, model building, controller development, and commissioning before moving onto deployment (see Figure 3). As a result, many companies were faced with lost margins during the lengthy deployment phase and disruptions to the process for gathering data to build APC models. Additionally, this process required highly experienced users to build and sustain controllers.

Some companies today have moved beyond the traditional methods to a more advanced technology by integrating adaptive process control with APC technology. With adaptive process control, these four phases have been combined into one, allowing users to experience faster deployments and sustained benefits through continuous model updates in the background with no disruptions to the process (see Figure 4). In addition, these tools enable more and less experienced users to deploy and sustain APC controllers, which can save time and money within the organization.

Advanced APC technology also helps companies sustain benefits. In the past, any changes to the process or equipment after deployment would require re-identifying the model, which would involve costly step testing and would be handled as a project. However, with adaptive process control technology integrated into today’s APC solutions, sustaining controllers is no longer handled as a project but rather as a continuous process.

Sustainability tools in these advanced APC solutions also include automatic bad-model identification. These models can be calibrated online, in a closed loop with no disruptions to the process. This makes sustaining benefits easier and cheaper. Additionally, maintaining APC controllers requires fewer resources, and as a result, controllers maintain peak performance, which in turn enables companies to deploy and sustain more controllers leading to a best-in-class APC program. 

APC is key to achieving operational excellence

The chemicals market is very competitive, volatile, and fast-paced. To maintain a strong market position, companies need to look harder and deeper into their equipment, production, and operations to ensure the production chain is optimized. At leading chemical companies, APC is a key strategic tool in the quest to reach higher levels of operational excellence. 

Tushar Singh is a product marketing manager Aspen Technology Inc. Kate Kulik is a senior principal business consultant at Aspen Technology Inc. Edited by Jack Smith, content manager, CFE Media, Control Engineering,


Key concepts

  • Advanced process control (APC) is a process control and optimization technology that takes into account the multivariable interactive nature of process units to reduce variability and drive the process to an optimum on a minute-by-minute basis.
  • The first step when implementing an APC program is to understand what process units or part of the units are good candidates for APC.
  • Some companies today have moved beyond the traditional methods to a more advanced technology by integrating adaptive process control with APC technology.

Consider this

In addition to the chemical industry, what other industries could benefit from advanced process control (APC)? 

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Original content can be found at Oil and Gas Engineering.