Digital twin for refinery-wide emission and efficiency monitoring

10:34 AM, 19th August 2020

Bharat Petroleum Corporation Ltd (Kochi Refinery) implements the digital twin concept as an innovative approach for real-time emission and efficiency monitoring and data acquisition.

By Chandrasekhar N, SarjeRao Chopade, Manesh D, Anoop Viswam, Rahul

The Business Challenge

Refineries at present face a difficult challenge of maintaining and improving the gross refinery margin, which calls for the enhancement of the refinery’s complexity. This is especially challenging in today’s economic uncertainty. At the same time, emission limits are already known, and refineries need to work in anticipation of new limits, which must be strictly followed. Achieving these two targets optimally requires data and models to evaluate the tradeoffs. Monitoring of refinery stack emissions for a complex refinery is increasingly challenging. While online analyzers are widely used for NOX, SOX, and CO, more information is required to ensure compliance. The introduction of dynamic limits established by India’s regulators presents the current challenge — moving emissions limits close to the operating value of units. Consequently, management pressure to ensure correctness of the analyzer value has intensified. Hence, the need for a real-time emission monitoring and data acquisition system was escalated as a priority at BPCL.

Furnace efficiency monitoring is crucial to attain cost optimization of fired furnaces. Traditional monitoring of furnace efficiencies with the Excel sheets is insufficient, as heavier duty, high-fuel consumption furnaces have become common. Every fuel-saving opportunity improves refinery margins and sustainability. Real-time, constantly calculated efficiency values have an important role to play to support management objectives. 

Need for a Refinery-Wide Emission Model

Bharat Petroleum Corporation Limited-Kochi Refinery, Ambalamugal (BPCL-KR) has an installed capacity to process 15.5 Million Metric Tonnes Per Annum (MMTPA) crude oil. BPCL-KR is executing currently a green field petrochemical project ‘Propylene Derivatives Petrochemical Project (PDPP),’ introducing more complexity. Excel-based workflows could no longer handle refinery emissions, heater efficiencies and stack emissions. It was clear that a modeling-based approach was the right answer. 

Challenges Facing the Environmental Team at Kochi

Typical continuous emission monitoring systems in use at plant sites are analyzers, which provide continuous information, but only at limited points, and are not predictive. This creates a critical gap as emission rules get more stringent and plants more complicated.

Challenges presented by the organization’s needs are as follows:

  • Validating correctness of the emission values provided by analyzers
  • Increased business need for accuracy of analyzer values due to regulations
  • Reliability of the theoretical values predicted
  • Prediction of actual emissions with fuel switching
  • Availability of historical data for CO2 emissions and the need to assess progress against the Paris climate change scenario of the carbon footprint
  • Direct evaluation of implemented energy-saving schemes with furnaces
  • Scenario planning for quantitative emission proposed by statutory agencies
  • Time savings for HSE personnel via a simple, intuitive emission data bank
  • Verification of O2 analyzer for reliability and accuracy of heater efficiency calculations 

Emission Digital Twin Model Concept  

BPCL-KR’s environmental team conceived of a refinery-wide emission model based on the current requirements. This project was initiated February 2019. Within six weeks, the BPCL team created an operating-validated Aspen HYSYS® model that computes steady state emission calculations. AspenTech worked with the BPCL, beginning in April 2019, to achieve online implementation of the validated model leveraging Aspen Online technology which accesses the correct online data and runs the model on scheduled intervals. With the joint BPCL-KR – AspenTech team, the Refinery-wide emission model was completed on August 3, 2019. After successful model updates and the accurate mapping of each area, the values obtained from this model closely matched the analyzers. An added benefit: parameters such as excess air helped to improve margin along with emission reduction. The following software comprises the emission model: the first phase of the project, already complete, identified significant scope for expansion and utilization in the field of cross-verifications: Aspen HYSYS for refinery digital twin, Aspen OnLine® for the online real-time deployment, Aspen InfoPlus.21® for big data storage and aspenONE® Process Explorer for the visualization dashboard.

 

Refinery-Wide Emission Model: Brief Overview

Digital twin models, which are first principle-based and thermodynamically validated, represent an alternative to sensor and testing approaches and accepted by many environmental regulatory bodies globally for monitoring and reporting on air pollution levels. The BPCL emission model is an innovative approach which estimates emissions instantaneously by picking up live values from the process historian and calculates required air emission levels with models based on thermodynamic first principles resulting in higher accuracy and benefit than traditional approaches, and enhanced ability to react in operations. The Aspen HYSYS platform was used to develop the emission prediction model.

Twenty-six heaters, six boilers, two sets of FCC regenerators, four sets of Claus reactors, and three sets of HRSGs were modelled together to make up a single Aspen HYSYS model flow sheet, thus composing a single model with higher accuracy. This then keeps providing current plant measurements through a live data connection with BPCL’s Aspen InfoPlus.21 historian, this data calibrates the model, and the model calculates pertinent emission values, which are written back to the historian (Refer Figure D). The values are displayed on Aspen InfoPlus.21 graphics dashboards with a simple, powerful and intuitive layout. Access to live values and trends are also made available.

Comprehensive Scope of the Digital Twin Model

The total emissions estimation involves some complexity because of the large scope of the network of heaters, boilers, regenerators and incinerators. Each heater has been carefully modelled in HYSYS using the best practices and distinct approaches for FCC and SRU stacks. Another challenge for effective model building is NOx and CO representation. The NOx generation mechanism is so complex that no practical manual solution other than analyzers can be used. The NOx generation mechanism has three kinds:

1) Thermal NOx

2) Prompt NOx 

3) Fuel NOx

Out of the three, fuel NOx is a minor part that can manually computed. Other two generation mechanisms are beyond manual computation. This HYSYS model has the capability to predict NOx and CO emissions from stacks based on excess O2 generation using reliable prediction curves. Since the model provides good results in the case of excess O2, this is a reliable method for prediction, based on standard curves. The curves can be remodeled based on actual emission data obtained through manual sampling or analyzer reading. Similarly, the model predicts CO emissions.

Additionally, we obtained accurate flue gas volume which is also helpful in mass emission calculation.The heater efficiency is calculated in the model using the actual specific heat quantity, based on parameters of amount of fuel and air taken. The efficiency can be predicted live using Aspen OnLine and output is available in historian trends.

One important aspect of this system is that it can give current fired energy of the refinery based on current fuel mix. This tool, combined with emission figures, can lead the planner through a more enlightened way of emission-based crude optimization.

How Does This Digital Twin Solution Support Sustainability and Digitalization Plans for the Refinery?

This digital twin system is a highly accurate way to understand the refinery’s performance on a day-to-day basis with respect to air emissions, including carbon emissions, and supports energy efficient operating strategies in the reactors.Currently, BPCL-Kochi is planned on a monthly basis using the Aspen PIMS planning system. Now, with this accurate emissions information, that parameter can be incorporated in the refinery planning, yielding increased optimization of margin, emission compliance, and carbon reduction.

Emission numbers have already been cross verified with calculation sheets and can be incorporated and utilized in crude procurement plans.The digital twin emission model can support sustainability plans like carbon neutrality and sulfur reduction. Additionally, it can suggest better fuel mix for the refinery that can reduce sulfur emission with targeted GRM. As the fuel mix decisions are so dynamic in a complex refinery, digitalization can only provide stronger results. 

Typically, analyzers are not transparent. They provide concentration values in mg/Nm3 and do not refer to actual quantity emitted unless volumetric flow rate is being measured in parallel. This digital twin model provides a much better option to broadly make available values in quantitative as well as concentration terms. Quantitative values better support sustainability plans as BPCL sets milestones for emission reduction.

Apart from man-hour savings, digitalization makes emission accounting simpler, more target-oriented and more productive. In our experience to date with this model, BPCL Kochi is able to maintain enough cushion in emission levels below regulated amounts which can be converted into margin benefits with a better choice of fuel mix.

Conclusion

Based on historical data run to validate this model, BPCL has found that the model computed and predicted values that are reliable for operating and compliance purposes. It is particularly valuable in cases with refineries which are using a different nature of crudes. This modeling approach is the only reliable and easy-to-implement source of comprehensive, refinery-wide emission data as it considers fuel composition along with mass flows.

This collaborative approach between AspenTech and a refinery site can be implemented in any refinery as it uses standard off-the-shelf software currently used in refineries globally. This represents a major breakthrough in emission monitoring and optimization of furnace operations. Widespread use of this approach will go a long way toward improving refinery margins, reducing emissions and decarbonizing refineries.

 

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