Energy management: raising profitability, reducing CO2 emissions
Case studies show how lower energy demand via revamps and improved maintenance can save money and abate CO2 emissions in refinery operations
Claire Weber and Thomas Yeung
Hydrocarbon Publishing Company
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Despite the failure to establish a binding agreement at the summit in Copenhagen, Denmark, in December 2009, climate change legislation and proposals are still being pursued by many nations around the world. The apparent uncertainties have not concealed the key deadline just ten years away, as most OECD nations have firmly committed to slashing CO2 emissions by 17–20% from 2005 levels by 2020. In mid-July 2010, French, German and British ministers proposed to significantly increase the EU’s targeted reductions in greenhouse gas (GHG) emissions to 30% by 2020, from the original goal of 20% by 2020.1 Even China, the world’s biggest emitter, has shifted away from its earlier opposition and now ensures that its emissions per unit of economic production will decline 40–45% by 2020. Therefore, refineries are expected to reduce their carbon footprint. As a recent industry survey by Hydrocarbon Publishing has indicated, many oil companies have already implemented CO2 emissions reduction strategies.2
In the past, energy conservation has been a cyclic area of focus. In the 1980s, high fuel prices forced refiners to look at ways to save money through energy management. At that time, the quick fix was the method of choice and, as fuel prices stabilised, improving energy efficiency was put on the back burner. Today, the drive to increase profit margins while reducing emissions has made energy management fashionable again. A common misconception about this area compares the strategies to make refineries energy efficient to low-hanging fruit. Many believe that, during the 1980s, the most effective strategies that would be easy to implement were acted upon, and now there is no room left to grow. This, however, is untrue. New techniques, inventions and strategies are currently available to maximise energy efficiency and to reduce GHG emissions. Further-more, many of the early strategies implemented in the past have not been sustained and no longer serve to maximise efficiency in their particular applications. This article focuses on options to reduce energy waste in three major areas of the refinery: process heat distribution, waste heat recovery and steam distribution. Although electricity is another type of energy distribution in refineries, it only accounts for around 5% of an average refinery’s energy requirements and will therefore not be addressed further in this article.
Distributing heat more efficiently throughout the refinery is a very important part of improving overall plant efficiency and reducing GHG emissions. In every refinery, multiple streams of feedstock and product are pumped from unit to unit and are frequently heated and cooled. This heating and cooling requires large amounts of energy, but by integrating the streams that need to be heated with those that need to be cooled the additional energy input required can be greatly reduced. The more effectively these streams are integrated through heat exchanger networks (HENs), the less energy input is required. Therefore, there are two areas to focus on when trying to improve overall heat distribution efficiency: improving the efficiency of the individual heat exchanger; and improving the efficiency of the HEN.
One of the major factors affecting the energy efficiency of a refinery’s heat exchangers is fouling. Fouling decreases heat transfer and efficiency in furnaces, boilers, heat exchangers and other process units by forming a layer of material with low thermal conductivity on heat transfer surfaces.3 In order to overcome this decrease in heat transfer efficiency, more fuel is required. The excess fuel increases both energy costs and CO2 emissions. The effects of fouling have been estimated to increase energy requirements by 12.3K Btu/bbl of crude processed (12.98 MJ/bbl).4 For a 100 000 b/d refinery that translates into about 426 B Btu/y (450 TJ/y) of wasted energy and around 23K mt/y of extra CO2 emissions. Overall, fouling costs refineries in the US more than $2 billion each year and refineries around the world a total of about $11 billion/y.5,6 There are four options to reduce fouling and increase heat exchanger efficiency: retrofit or modify existing systems; use anti-foulant additives; install new heat exchangers; and use software for troubleshooting poorly performing equipment and/or selecting new exchangers.
Due to large variation in such factors as the cause and location of fouling problems, the current type and arrangement of heat exchangers, the cost of fuel and the cost of emissions, the economics and practicality of implementing any of the measures discussed will vary from plant to plant. Therefore, a thorough study must be performed before any measure is implemented. Software programs that identify fouled heat exchangers and programs that assist users with picking the most suitable type of heat exchanger for a particular application may be helpful in such studies.
Table 1 compares the relative cost and benefits of several options for reducing CO2 emissions by maintaining or retrofitting existing heat exchangers. There are many low- to medium-cost options for improving the operation of heat exchangers in refineries, especially for those prone to excessive fouling.
There are also several options for new heat exchanger designs that improve heat transfer and decrease fouling, including spiral tube, twisted tube, spiral plate and plate heat exchangers (PHEs). Although different types of heat exchangers may have niche applications, PHEs seem to be the most promising for most applications in the refinery. Due to the higher heat transfer rate, lower fouling rate and close allowable approach temperature, refineries should thoroughly consider PHEs when replacing or installing new heat exchangers. In new applications, the costs of a PHE may be only marginally higher than the cost of installing new shell-and-tube (S&T) heat exchangers. In some cases, such as when two PHEs can do the work of four S&T exchangers, the cost for using the alternative heat exchanger is actually less than the capital cost required to purchase and install a new S&T heat exchanger(s).
The key to improving the energy efficiency and reducing the CO2 emissions associated with a heat exchanger network as a whole is through process integration. By integrating the process streams that require heating with those that require cooling, less hot and cold utilities are needed. This can substantially reduce the energy requirements of a plant. Many different methods have been developed to optimise heat integration, including mathematical programming approaches and pinch analysis — a thermodynamic approach. The pinch design method has been a valuable tool since its inception, and further advances and adaptations have made it the primary tool for optimising heat integration. Industry experience has shown that energy requirements can be reduced 10–30% by applying pinch technology to optimise heat integration. For refiners who have not performed a pinch study recently, this is one of the key ways to reduce energy use and CO2 emissions in the plant.
Waste heat recovery
The implementation of efficient waste heat recovery systems is another very important factor for minimising CO2 emissions from a refinery. For every one megawatt the refinery recovers from waste heat, about 2900 mt/y in emissions savings is realised.7 Depending on the temperature and mass flow rate, waste heat can be used in many different ways. For example, waste heat in the 750–2250°C (1382–4082°F) range is often used for generating steam; waste heat in the 400–1000°C (752–1832°F) range may be useful for heating water; and waste heat in the 250–500°C (482–932°F) range can be used for space heating.8 Table 2 shows the temperature ranges for various sources of waste heat and the associated energy lost per barrel of crude processed.9
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