Oct-2006

Refinery CO2 challenges: part IV

The strategic energy review approach is summarised and results from recent studies conducted at a European refinery are discussed

Vicente de Pablo Bruna, CEPSA Gibraltar Refinery
Jem Hart and Allan Rudman, KBC Process Technology Ltd

Article Summary

European refineries are facing the challenge of reducing CO2 emissions to meet national targets set by the Kyoto agreement. For most refineries, this will require changes in operational practices and significant capital investment in energy-saving projects. The challenge is how to maximise CO2 savings with optimal capital investment.

KBC’s strategic energy reviews (SERs) combine an experience-based optimisation method with a systematic Total Site approach, to provide a complete spectrum of energy benefits, ranging from immediate-return to long-term investment projects. The outcome is an investment RoadMap to define the optimal energy improvement strategy.

An SER consists of four key stages that lead to a site having an optimised energy system. Figure 1 shows a simple schematic of these stages.

Benchmarking and gap analysis
For the benchmarking and gap analysis stage of the SER, utility consumption data (fuel, power and steam) is collected for each process unit (including off-sites), to determine the site energy consumption. The refinery energy performance is assessed in terms of the best technology (BT) index for the site. The BT index is the ratio of the refinery’s total energy consumption to the BT allowances (standards) for each process unit and the off-sites.

The BT standards have been developed by fundamental analysis and design studies on individual process units. They take into consideration key process parameters affecting energy consumption, such as the actual feed quality, conversion and fractionation performance. KBC believes that an energy performance of 100% BT is achievable in a grass-roots unit built today. Such a unit has an economically justifiable level of energy efficiency, and is supported by a highly efficient steam and power system.

The energy-efficiency distribution diagram of the more than 100 refineries surveyed worldwide by KBC is shown in Figure 2. The survey shows that a pacesetter refinery would have a BT index of 125%. So, any process unit with a BT index greater than this will have inefficiencies and potential for energy saving. Gap analysis is used to quantify where the inefficiencies are and target energy savings.

The gap analysis considers three main criteria:
• Power generation efficiency The BT power generation efficiency is 80% or more, with power being generated by a mixture of back-pressure steam turbines and gas turbines
• Fired heater efficiency A BT furnace has a stack oxygen content of <3% with an efficiency of 92%
• Heat integration Pinch analysis is used to determine the target minimum energy consumption, with an optimised approach temperature in the heat recovery system. For example, a BT crude distillation unit would have an approach temperature of 20°C.

The benchmarking and gap analysis highlight the most efficient and inefficient areas of the refinery. The analysis will also show which of the process units’ energy improvements will have the biggest impact on energy and CO2 reduction, as shown by an example in Table 1.

An example of the impact the BT index might have is shown in Table 2 for a typical conversion refinery that is self-sufficient in electrical power.

For the example in Table 2, the CO2 emissions would be around 300 t/h (~2 500 000 t/y), depending on the type of fuel being burnt.1 An improvement in the BT index of one percentage point will result in an energy saving of 4.1 Gcal/h, with a corresponding CO2 saving of 1.4 t/h (~11 400 t/y).

Operational review
Most refineries try to maintain efficient operation to minimise energy consumption. However, there is always scope for further improvement. The idea of this stage of the SER is to assess the benefit of operational improvements. Two steps are used to aid the process:
• A PFD review. This takes place for each process unit and consists of a brainstorming session between engineers and operators, with the primary objective being to identify scope for energy improvements from operational changes
• A site walk-around to assess site operations and identify inefficient operation of major equipment items (furnaces, turbines, compressors). Levels of housekeeping are also determined and any poor practices reported (insulation integrity, steam trap functionality, stem leaks).
Typical outcomes for this stage are:
• Improved furnace operation, with better monitoring and control of the excess oxygen
• Opportunities for hot transfer between process units
• Optimised heat exchanger cleaning cycles
• Increasing the high-pressure steam header temperature to maximise power production
• Switching from small, inefficient turbines to electric motors
• Process changes (for example, optimisation of distillation columns’ reflux rates).
Energy savings from operational improvements can give CO2 reductions of 1.0–4.0%, as shown in Table 3.

Process/total site review
Once the operational changes necessary to minimise energy consumption are identified, given the existing infrastructure, the next stage is to determine the structural improvements required for further energy reduction. KBC uses the proprietary Total Site approach2 to identify energy optimisation opportunities. This consists of two elements: