Aug-2020

Molecular management for refinery-petrochemical complexes

Designing a digital twin for process simulation of an integrated complex enables enhanced molecular management to deliver higher margin.The future comprises a world of fuels refineries, refinery- integrated petrochemical complexes and crude oil to chemicals. Through the energy transition, the latter two will no doubt be more resilient toward future demand- and/or supply-side dynamics.

JITENDRA CHELLANI and SACHIN SRIVASTAVA
KBC (A Yokogawa Company)

Article Summary

The potential for incremental improvement with increased conversion capabilities of a fuels refinery with added petrochemical integration is $1.5-2/bbl of crude processed. The value gained from effective molecular management is significant. Key objectives of molecular management for the overall complex are:
- Improvement in gross product worth by maximising high value products
- Reduced cost of feedstock by replacing expensive imports with low value streams

However, with the increase in complexity of refinery-integrated petrochemical complexes, a gap has developed in the tools needed to effectively drill down to the stream carbon number level on a continuous basis. Traditional tools used for refinery optimisation rely on bulk properties such as cut point for stream optimisation. Such methods have been found to have limitations in fully achieving the objectives of molecular management. The latest process simulation tools address this by enabling detailed carbon number breakdown from crude assays through to blending and petrochemical units, for the whole integrated complex. These enhanced capabilities enable identification of site-wide optimisation opportunities across the integrated complex, as well as ongoing sustainment of benefits through real-time monitoring and re-optimisation.

Why manage molecules?
To recognise the need for molecular management and carbon number level simulation, it is important to understand the difference in nature of refining and petrochemical processes. The purpose of refinery naphtha processing units is to maximise octane barrels which can be blended into gasoline. Isomerisation and dehydrocyclisation reactions improve octane, with thermal cracking a side reaction that is minimised to prevent yield loss. However, for steam cracking in petrochemicals, thermal cracking of naphtha is optimised to produce olefins.

Due to differences in the units’ reactions, selection of molecules to be processed is extremely important. Replacing molecules from one process unit to another can improve yields from both the specific units as well as reducing operating expenses such as energy consumption, other utilities, catalyst, and chemicals.

The yields from a steam cracker can vary extensively depending on the feedstock carbon number and different isomers (see Figure 1).

Yield of ethylene and propylene is expected to reduce while yield of heavier products is expected to increase with an increase in the feed carbon number. Even for feeds with the same carbon number, cracking yields change with changes in C/H ratio, for instance paraffinic feeds have higher ethylene yield. Different feeds can be cracked separately or co-cracked, all having an impact on the cracking yields and furnace run length. Aromatics components in the feed rarely crack to produce ethylene and propylene, but these may be too costly to separate prior to processing.

The profitability of the aromatics is driven by the reformate yield and distribution of components in the reformate. Higher yield of xylenes over benzene is preferred from the aromatics, which could be achieved by selecting the right feed for the naphtha reformer.

Selection criteria for different feeds in refinery process units depends on the predicted octane barrels produced by feed components.
Figure 2 explains typical preferences which can be used to select feeds for different process units and product blending:
-  Lighter hydrocarbons like C2 components in off-gas and LPG are preferred feedstock for a steam cracking unit as these molecules provide higher yield of lighter olefins.
-  First preference for routing C5/C6 molecules should be the steam cracker due to the significant premium for light olefins over gasoline. C6 naphthenes and benzene in the reformer feed will contribute to benzene product for an aromatics reformer and can produce off-spec product for a gasoline reformer.
-  C7s tend to crack more in the ISOM unit compared to C5/C6 and therefore should be minimised. Routing of C7 molecules to the aromatics reformer is preferred over the gasoline reformer. This routing leads to a reduction in the A7/A9 ratio in a transalkylation unit and improves octane barrels of gasoline pool.
-  C7+ components in heavy and medium naphtha can also be used for middle distillate blending. If C7+ components cannot be processed in any of the process units, they can be blended in middle distillates diesel rather than selling them as open grade naphtha.
-  C8s are preferred feedstock for an aromatics reformer as they reduce the throughput of most of the process units in the aromatics and recycles as well. This reduces cracking to light ends and the opex of the units, and improves p-xylene yield.
-  C9s in aromatics reformer feed reduce the A7/A9 ratio of a transalkylation unit which improves the yield of xylenes and reduces benzene yield.
-  C10+ components are generally not preferred in aromatics, but these can be processed in a steam cracker.

Apart from the reactions and yields, feed selection criteria strongly depend on opex and the relative prices of fuels compared with petrochemicals. The latest process simulation tools are critical for estimating optimum steam or molecule routing based on stream composition. Molecular management is not limited to optimisation of refinery naphtha only, as the optimised routings are applicable for streams from the petrochemical complex as well.

Challenges
Quality parameters measured and analysed by the refinery engineers are usually based on bulk properties. Distillation and density are considered the most important properties for refining units. Though gas chromatography is performed for selected streams in naphtha processing units, components routed to refinery products are seldom analysed. The molecules lost in jet from the crude unit and the composition of hydrocracker naphtha are occasionally analysed in the refinery. On the other hand, the focus for petrochemicals is usually on pure components rather than bulk properties. Some of these components can be lumped together for monitoring purposes but detail to the level of isomers is required for a few units. For example, N+2A is used as a feed quality indicator for a gasoline reformer, however C7/C9 distribution is important for an aromatics reformer.

Simulation tools for refinery and petrochemical units are developed based on the information available — bulk properties for a refinery and composition for petrochemicals. Integrating these tools to create a complex-wide process model has been a challenge for process engineers in the past.

Tools based on linear programming (LP) are a solution for bigger optimisation problems such as crude selection, however they are rarely configured for molecular management. Even when they are used, LPs have their own limitations. Organisations focused on refining operations use comparatively simplistic techniques to simulate petrochemical units in LPs; petrochemical operators typically start optimisation at the naphtha feed and do not simulate component based assays and the fractionation efficiency of refinery optimisation.