Capturing APC benefits from secondary units

Implementing APC applications on secondary units and networks are significant from both a margins increase perspective and an operational excellence perspective.

By Stefano Lodolo and Tushar Singh

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Advanced Process Control (APC) has been used in the process industry and in various forms for many decades. From DCS-based primary control schemes and computer-based Multivariable Predictive Control (MPC), to a wide variety of process units such as atmospheric and vacuum distillation units, conversion units (hydrocracker, FCC, coker, visbreaker), hydrotreaters, gasoline reformers and often some other units like isomerization, steam reformers and distillation trains. While many organizations focus only on major process units, the best in class ones expand APC footprint to secondary units to achieve additional benefits. This may include APC for environmental emissions control, fuel gas network control, sulphur complex control, steam networks control, H2 networks control and more.

The benefits of implementing APC applications on these secondary units and networks are significant from both a margins increase perspective and an operational excellence perspective. However, due to a lack of understanding of achievable benefits, shortage of APC resources and gap in APC technology, among other reasons, some organizations do not expand their APC footprint, leaving significant benefits on the table.

Environmental emissions control

In a typical refining or petrochemical site both Fuel Oil (FO) and Fuel Gas (FG) are combusted in some furnaces or boilers. Analyzers are available at furnace level (typically O2, CO) while others are available at stack level (typically SOx, NOx) with multiple furnaces sending flue gas to the same stack.

Ideally, facilities would have a system in place that would measure the environmental limits in the furnace and stack level, however due to the local, regional and specific country regulations, the process to respect these limits can be extremely complicated. Limits tend to become more and more stringent over time, they may apply for dust, CO, NOx, and SOx for both concentration and mass and they sometimes apply to single furnaces, boilers, single stacks and/or the overall site. Moreover, these limits are not to be applied “as-is” and rules like “(97% of 48h averages) < (LIMIT * tolerance)” often apply. In addition, to make things more complicated, limits are actually changing depending on the fuels mix, e.g. the FO/FG ratio. 

Given the above complexity, operators typically only control the real-time measure, taking a huge safety margin compared to what an APC system can do. This leaves on the table a relevant amount of money that could be measured in millions of dollars per year. For instance, APC systems have been successfully designed and commissioned as part of a bigger scope to address the specific problem of emissions limiting the plant load for major units like crude columns or ethylene crackers. Moreover, controlling the environmental limits (not just monitoring the environmental limits) in closed-loop provides owners and operators a distinctive advantage in managing environmental constraints and regulations with ease.

Fuel gas network control & optimization

Fuel Gas Network control is another very difficult control problem within the refineries and petrochemical plants. Today, it is common to see sites with an unstable Fuel Gas Network which becomes a major issue, as any upset can affect the whole site, impacting both operations and the site energy spend. However, with APC technology, these sites can benefit from a stabilized Fuel Gas Network and optimized energy management.

A Fuel Gas Network can be really complex with multiple headers, KO drums, flares, gas recovery compressors, LPG vaporizers and more. Off-gas flows change continuously (particularly when the units load changes) and if enrichment gas with a high heating value is added to achieve the volume balance, i.e. to control the pressure, the heating value of the fuel gas increases. The COT controllers on the furnaces will cut back on consumption which will upset the volume balance, and the enrichment gases will also be cut back. This behavior keeps going, leading to a self-propagating cycle with the system possibly becoming unstable. Fuel gas instability can lead to units having to cut back load or result in flaring events - both very undesirable situations.

Fuel Gas pressure and calorific value are interactive and pose a highly non-linear problem that can only be addressed using the proper tools and technology. Volume balance models are integrating processes as pressure builds up as a ramp and exhibit strong non-linearity. Model gains can flip signs as the quality effect of the enrichment gas depends on the current Fuel Gas Network’s calorific value.

Download the 2016 State of Technology Report on PLCs, PCs and PACs

Adding makeup gas can increase or decrease the overall calorific value which may change over time depending on the specific makeup being used. Adding additional makeup gas has less of an affect as it approaches the current quality. There is no way of easily calculating how much enrichment gas should be added and for how long, as the consumption changes with additions. However, by implementing APC applications and creating a Fuel Gas optimization solution, organizations can achieve major results. For example, APC can: 

1. Dramatically reduce the variability in the refinery Fuel Gas system

- Simultaneously stabilize pressure and calorific value
- Stabilize furnace operations
- Stabilize process unit operations

2. Minimize overall fuels cost

- Typically maximize use of Fuel Oil
- Minimize, whenever possible/convenient, use of enrichment gas (LPG/Propane/Butane/CH4)

3. Strictly respect the furnace’s/stack’s environmental limits while making money

- Closed-loop control of emissions; not just emissions monitoring
- Minimize fuel cost until the process unit’s constraints or emissions limits become active

4. Help other APC controllers to maximize throughput and overall profit

- Consistently push local and overall constraints, taking the benefit of a more stable Fuel Gas Network

5. Minimize/completely eliminate routine flaring which is a cost and impacts the environment

6. Minimize blow down gas recovery compressors consumption

7. Reduce the Energy Intensity Index (EII)

- Improve furnace efficiency through more stable firing, enabling further reduction in excess air

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