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May-2017

A broader view to improve 
energy efficiency

A project to improve a FCC unit’s energy efficiency took into account surrounding process units to expand opportunities for saving energy and utilities.

NICOLAS LAMBERT and FILIPA GOUJARD
Axens
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Article Summary
Greenhouse gas (GHG) emissions have become a growing concern for many industrialised countries over the past few years, as confirmed by the Paris Agreement in December 2015.

Beyond the specific issues of GHG and general environmental considerations, there is a global tendency for improved energy efficiency. Indeed, whether the price of energy is high or low, controlled and reduced energy consumption will naturally improve operators’ margins.

In the particular case of Russia, Presidential Executive Order number 752 was passed in September 2013, stating that the volume of GHG emissions will have to be reduced by 2020 to 75% of the baseline set in 1990.

As indicated in the International Energy Agency’s 2015 World Energy Outlook (Special Report on Energy and Climate Change),1 Russia has indeed already drastically improved energy intensity between 1990 and 2014 (see Figure 1). Yet there is still a large potential for improvement in general and more particularly in the refining industry which represents about 5% of total energy use (including industry, transport and buildings).1
This article will deal with the methodologies available to refiners to assess the thermal efficiency of their process, focusing on the case of the fluidised catalytic cracker (FCC) and its pre- and post-treatments, finally covering a revamp case of a refinery in the CIS area.

Evaluating energy efficiency
Pinch analysis is a systematic methodology for energy saving based on thermodynamic principles developed by Linnhoff March in the 1970s. It needs limited information about material (flow rate) and heat balance (temperature T, specific heat capacity Cp and enthalpy H). This method is useful to compare two networks, identify inefficient exchangers or assess the minimum energy (cold/hot utilities) required by the system, thus the maximum potential for improvement of a given scheme (the target). Pinch analysis never provides a solution but indicates routes to improvement. Moreover, this methodology cannot take into account specific process or operating constraints. As such, no optimised heat exchanger network (HEN) is obtained.

Pinch analysis involves a graphical resolution through the building of ‘composite curves’. Composite curves aggregate, on an enthalpy/temperature graph, all of the heat availability in the process (the ‘hot’ curve) on the one hand and all of the heat demands or heat sinks (the ‘cold’ curve) on the other hand. The cold and hot composite curves therefore run more or less parallel. At the point where they are the closest to each other, the ‘pinch’ temperature is defined and the difference in temperature at this point is called ΔTmin (see Figure 2).

With such a graphical representation, it is possible easily to identify design issues or poor exchanges. Indeed, thermal transfer will be impossible for instance if the cold curve is above the hot curve. In addition, three ‘golden rules’ have been defined to ensure the best efficiency:
• No external cooling above the pinch
• No external heating below the pinch
• No heat transfer across the pinch.

Properly integrated process units routinely exhibit ΔTmin in the range 30-40°C. (This can vary depending on the process considered, site utilities conditions, and so on.) A target can be set with a lower ΔTmin, thus targeting a better efficiency. The cold composite curve is therefore translated until this new ΔTmin is reached. The new relative position of the cold and hot composite curves will then define the minimum hot and cold utilities consumptions necessary, and therefore the ultimate potential for energy savings. The optimum ΔTmin is a trade-off between investments (low ΔTmin) and operating expenditures (high ΔTmin), yet the pinch methodology will provide neither the optimum ΔTmin, nor the associated best possible optimised HEN.

Indeed, as previously indicated, pinch methodology provides only indications, for a given situation, that some exchanges are to be reviewed, not taking into consideration potential impossibilities due to technical limitations or process constraints. It is therefore necessary to use more sophisticated tools and this is why an in-house tool has been developed with IFP Energies nouvelles (IFPEN) in order to generate optimised HENs.

This advanced tool needs much more data than pinch analysis and can be coupled directly with process simulators such as PRO/II. It is therefore possible to evaluate large-scale units with added features:
• Bypass requirements can be set, in case the process needs to be run under different operating modes (seasonal operation, varying objectives of feed cocktail, for instance) or to take into account transient phases.
• Restrictions on approaches can be set.
• Some exchanges can be forbidden (high pressure fluid exchanging against low pressure fluid, very low duties, and so on).
• Number of exchanges can be limited to limit the complexity of the scheme (and associated capital expenditures).
• The target is generally defined as an energetic optimum but can be tweaked to take into account unit costs (opex), GHG regulations, minimum investment (capex), reduction in the consumption of a particularly expensive utility, or any combination of these.

Consequently, the more data that is available, the larger the number of possibilities offered, and ultimately the better the HEN optimisation. This methodology has been applied to a FCC complex and the results are developed hereafter.

The FCC complex
Nowadays, a FCC unit can be thermally integrated to a large extent, especially when the unit is designed with a propylene recovery unit (PRU), which will allow good usage of low heat level streams to reboil columns such as a depropaniser and a deethaniser (see Figure 3).

As such, it becomes fairly common to reach ΔTmin under 30°C while the industry standards are in general in the range 30-40°C. This seems to be the best performance achievable with conventional process equipment. Therefore, in order to get around such a limitation and seek even better performance, we have either to consider unconventional process or heat exchange equipment (generally available at a higher cost or with limited warranty due to reduced operational feedback) or expand the boundaries of the study. The latter will allow including more fluxes, thus a larger potential for further improved thermal integration as previously indicated.

Therefore, instead of focusing only on the FCC unit, we shall have a look at the ‘FCC complex’. The FCC complex can for instance be defined as the chain of processes involving FCC feed hydrotreatment (cat feed hydrotreater, CFHT) and any or all of the post-treatments of the FCC effluents.

Each element constituting this chain presents specific issues. In terms of flexibility:
• The FCC unit can be operated in different modes, resulting in fairly different yields, for instance maximum gasoline and maximum propylene modes of operation.
• The hydrotreating catalysts will age over time and exhibit different performances between start and end of run (SOR, EOR).
• Operating conditions can be tweaked to maintain optimal catalyst activity or adapted to the desired mode of operation, resulting in turn in different demands for utilities consumptions.
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