Petrochemical complex digitalisation for greater efficiency and emissions reduction

Concepts of automation and data exchange in manufacturing have propelled both upstream and downstream sectors into the 4.0 world, gradually extending to petrochemical plants.

Philippe Mège and Michel Molinier

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Article Summary

Numerous benefits are expected from digitalisation transformation, including real-time business management and associated efficiencies, minimised environmental impact, and improved sustainability and safety. In petrochemical complexes, process licensors and catalyst suppliers play a key role in accompanying this transformation as they provide the digital tools complementing their proprietary technologies, implement tailored users features such as diagnostics and alerts, predictive functions and improvement suggestions, as well as cybersecurity solutions to ensure the confidentiality of their real-time exchanges with operators.

Digitalisation options must come at a competitive cost to ensure the benefits largely surpass the additional investment. Process unit remote monitoring by licensors and catalyst suppliers used to be a challenge due to the time lag between data collection, data submission, data interpretation and feedback transmission, and time zone differences. With real-time data display and exchange of operating status viewgraphs for a unit or an entire complex, Axens is in an ideal position to assist operators in maximising the efficiency of their processes, with benefits for both users and technology providers.

Against this backdrop, it benefits to show the advantages of digitalisation in an aromatics complex, where processes are inherently and strongly interconnected. Examples of performance visualisation by various units of measure, alert functions, optimisation tools, and performance prediction are presented together with yields improvements and ensuing benefits in commercially operating plants. The impact of digitalisation on GHG emission reduction is also detailed and discussed. Finally, the coming digital twin approach for a petrochemical plant is introduced.

Digitalisation for the petrochemical industry
Real-time business, real-time supply chain management and optimisation, production excellence, enhanced sustainability and compliance, and reduced risk to personnel safety and health were among the many expected benefits as industry 4.0 concepts caught up with the refining industry.1 These improvements are now extending to petrochemical plants, even if many chemical companies are still figuring out how to apply digitalisation to their business in the most effective way.2 Process licensors are in a unique position to accompany the digital transformation of petrochemical operations as they:
- Master the processes they licence, hence are in the best position to design digital tools for these processes
- Explain to users the possibilities and benefits associated with the digitalisation of their process units
- Supply cost-effective, user-friendly digital applications, easy to implement in existing or new facilities
- Provide valuable functionalities, including plant overview and unit-by-unit performance tracking, real-time economic evaluation, alerting functions, support to the planning department, operator skills management features, and so on
- Use real-time data analysis to communicate with end-users
- Offer tailored calculations, solutions, and optimisation options, including the ability to monitor the performance of closed-loop advanced process controls (APCs) and correct/readjust such APCs as needed
- Regard and handle digital monitoring as a requisite for the technologies they supply, as essential as online analysers, flowmeters, and thermocouples.
Figure 1 shows an example of a modern aromatic complex configuration. A naphtha stream is hydrotreated and split into light naphtha (LN) and heavy naphtha (HN) streams, with the latter feeding a continuous catalytic reformer (CCR). Benzene and toluene are extracted from the light reformate, while the non-aromatics raffinate is typically directed to the refinery gasoline pool or used as steam cracking feed or fuel.

The heavy reformate is sent to a xylene column where C8 aromatics are collected overhead and feed a xylene loop for paraxylene separation, ethylbenzene conversion, and isomerisation of para-depleted xylenes. C9+ aromatics are collected at the xylene column bottoms and further fractionated in a heavy aromatics column. C9 and C10 aromatics are recovered overhead of the heavy aromatics column and processed with toluene in a transalkylation unit to produce additional benzene and xylenes, while C11+ aromatics are collected at the heavy aromatics column bottoms and typically exported as fuel oil.

Aromatics complex: interconnected processes
The aromatic complex constitutes a textbook case of interconnected processes. Below are a few examples of process changes influencing other processes:
-    The naphtha splitter operation can be adjusted to retrieve more molecules from the LN stream and direct them via the HN stream to the CCR for additional benzene production. The CCR operation needs to be fine-tuned accordingly, not only to maximise aromatic products but also to minimise concurrent olefins make; further, the reformate splitter, as well as the extraction process, need to be able to accommodate such additional benzene product.
- Paraxylene adsorption utilisation at near full capacity heavily depends on the proper operation of the upstream clay treater and/or selective hydrogenation unit.3 Meeting olefins specification is critical for the C8 aromatics stream feeding the paraxylene selective adsorption unit, and thus real-time management of potential upsets in the olefins removal process is essential in modern aromatic plants.
- In facilities where xylenes isomerisation is split between gas phase and liquid phase processes, energy consumption is minimised when traffic through the liquid phase process is maximised. However, xylene loop ethylbenzene concentration increases with traffic through the liquid phase process, which reduces the overall loop efficiency. Consequently, energy consumption reduction, ethylbenzene conversion, and overall loop efficiency require real-time optimisation.
- The heavy aromatics column operation sets the nature and quantity of C9, C10 and possibly C11 aromatics that will feed the transalkylation process to produce additional xylenes and benzene. More heavy aromatics in the feed typically means higher xylenes production per pass and a higher ageing rate for the catalyst used in the transalkylation unit. Higher xylenes and lower benzene production or vice versa impact paraxylene recovery as well as benzene fractionation.
- Feed selection affects all units in the aromatics plant. The use of a process operating simulator (POS) for pre-screening of available naphtha streams allows overall complex simulation and prediction of product slate associated with different feed scenarios. The POS is an essential planning tool for fast decision making to respond to PX production demand as needed.

Real-time digital performance monitoring cases
Nowadays, data densification techniques such as machine learning enable the creation of soft sensors to compensate for lab analyses low frequency. Consequently, it is possible to determine — with the same granularity as based on actual process data — the compositions required for near real-time modelling. The following cases are from aromatic complexes currently in operation.

For example, in the naphtha splitter, LN detailed carbon breakdown can be continuously determined to estimate the total naphtha feed TBP curve based on carbon PONA composition. The splitter LN/HN operating cut point can be calculated, and a set of optimal conditions is proposed in real-time to operators in order to adjust the LN swing cut flow rates that can be directed to HN.

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