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Advancing export terminal technology: An optimized process for the refrigeration of cryogenic hydrocarbons

One of the greatest challenges facing the world is climate change, the aftermath of emissions of greenhouse gases into our atmosphere. Awareness of climate change and its causes is spurring the development of new methods to reduce carbon dioxide (CO2) emissions.

In support of the Paris Climate Agreement, India is committed to tackling this ongoing issue. The country’s balanced and comprehensive Intended Nationally Determined Contributions (INDCs)—a term used under the United Nations Framework Convention on Climate Change (UNFCCC) for reductions in greenhouse gas emissions, which all countries that signed the UNFCCC were asked to publish before the 2015 United Nations Climate Change Conference held in Paris, France, in December 2015—are proving that the recent decisions of the Indian government represent a quantum leap in its aspirations to face the challenges of climate change.

According to the International Energy Outlook 2016, world consumption of marketed energy will increase from 549 quadrillion Btu in 2012, to 629 quadrillion Btu in 2020, and to 629 quadrillion Btu in 2040—a 48% increase from 2012 to 2040. Oil remains the world’s leading fuel, accounting for 32.9% of global energy consumption, according to a BP study. Meeting the growing energy demand will present further challenges due to the constraints required to minimize environmental damage.

The Paris Agreement’s focus on energy, as well as the new policies in place, will impose greater restrictions on emissions of CO2. This will significantly impact the fossil fuel sector. The shift in policy has made its mark on the investment pattern in the energy sector—at a time when investment in upstream oil and gas has fallen sharply, roughly $1.8 T/yr of energy sector investment has been attracted to clean energy. The value of fossil fuel consumption subsidies experienced a tremendous reduction in 2015, dropping to $325 B from almost $500 B in the previous year. This reflects not only lower fossil fuel prices, but also a subsidy reform process that has gathered momentum in several countries.

A new approach

Fossil fuels continue to dominate energy supply, but the overall distribution of energy sources is changing in both share and investment. Mitigating climate change by reducing CO2 emissions is not an easy task for the conventional sector, as a significant monetary burden is imposed on oil and gas companies. An approach is needed regarding new technology that focuses on innovation, as well as on retrofitting existing systems with new combinations. Identifying and investing in technology that will reduce carbon footprint and, at the same time, generate profit is the best option for oil and gas sector companies. Oil extraction and refining are consuming almost 10% (approximately 9 MMbpd) of the total oil produced—the US Energy Information Administration’s (EIA’s) Short-Term Energy Outlook 2017 estimates that total oil production in 1Q 2017 was 97 MMbpd. This is equivalent to 0.055 exajoules per day (EJ/d), considering the calorific value of crude as 43 megajoules (MJ) per kilogram (MJ/kg), which is sufficient to light 6.2 B homes per month (15 W for 5.5 hr for 30 d), or 515 MM homes/yr.

The integration of renewable energy sources into the conventional energy sector can be a promising solution if the economics of both can coincide. The opportunities and applications for solar/thermal systems to fully or partially replace fossil fuels are explored here.

India’s oil refining capacity for 2016–2017 was projected to be 310.6 MMtpy, but the lack of accurate data regarding the total energy consumption for the country’s oil refineries exposes the negligence of the sector. The reason for including Indian refineries in the Perform, Achieve and Trade (PAT-2) cycle is their fuel consumption itself.

As per international benchmarks, the specific energy consumption (SEC) of an Indian oil refinery is 542 Kcal/l–872 Kcal/l (kilo calorie per liter), or 5.42%–9.27% of total production. This means that 5%–10% of the energy produced is used for the production process. This is not an insignificant amount and therefore deserves recognition. Energy efficiency enhancement and an increase of environmentally friendly energy resources (e.g., wind, solar, biomass) in the global energy mix offer a promising solution. Lower carbon emissions, increased long-term profitability, higher natural gas prices and the non-availability of natural gas in certain geographical locations are the main drivers for the adoption of renewable energy. Solar/thermal technology is used in only a few areas, such as in steam generation for enhanced oil recovery (EOR) applications in the Middle East and North Africa (MENA) region.

Within oil refineries, a good share of energy consumption is utilized in the name of “process heat.” The energy demand is met by the burning of conventional energy resources such as oil and gas. Globally, the refining sector ranks third in stationary CO2 emissions, as shown in TABLE 1. A typical refinery emits 0.04 metric t/bbl–0.049 metric t/bbl of CO2 in processing light to heavy crude, which indicates the potential and need for integrating solar/thermal technologies—such as parabolic trough collectors (PTCs), linear fresnel reflectors (LFRs), solar towers (STs) and non-concentrating collectors—that can result in considerable energy gain.

Energy consumption in refineries

Refinery operations generally consume energy in three different forms: heat, steam and electricity. For the same processing capacity, the amount of energy consumed changes depending on the refinery’s complexity—as the complexity increases, so do the process steps and energy consumption. It is important to understand the average consumption of any refinery to set a baseline for the study to quantify the reduction in consumption of conventional energy (FIG. 1 and TABLE 2).

Fig. 1. Energy consumption of refineries.

Fig. 1. Energy consumption of refineries.

Ninety percent of the total energy is used as thermal energy for the high-temperature processes: heating of feed (65%) and steam generation (23%). The average energy consumed in each process and their temperature requirements are highlighted in FIGS. 2 and 3.

Fig. 2. Specific energy consumption in different refinery units, Kcal/l.

Fig. 2. Specific energy consumption in different refinery units, Kcal/l.
Fig. 3. Temperature requirements of different sub-processes in a crude refinery (coking refinery configuration).

Fig. 3. Temperature requirements of different sub-processes in a crude refinery (coking refinery configuration).

When examining solar/thermal integration in refineries, it is important to consider some of those applications in detail, along with the understanding of the basic features of various solar/thermal technologies.

SOLAR/THERMAL TECHNOLOGIES

Fig. 4. Direct normal irradiation (DNI) world map (Source: DLR 2008).

Fig. 4. Direct normal irradiation (DNI) world map (Source: DLR 2008).

Concentrating solar/thermal (CST) systems use combinations of mirrors or lenses to concentrate direct beam solar radiation to produce forms of useful energy, such as heat, electricity or fuels. CST systems are mainly classified as either non-concentrating or concentrating systems. Direct normal irradiance (DNI) is the primary parameter in deciding the total energy available for the CST system. A global DNI profile is shown in FIG. 4, and can be used to analyze the solar/thermal potential of a particular geographical region.

A solar/thermal system combined with a thermal energy storage (TES) system is particularly effective, as it can store the generated heat and can be used when solar energy is unavailable. Heat transport and storage have become a focus of research and can provide solutions to a variety of challenges related to the intermittent nature of solar energy. The three types of TES are sensible heat storage, latent heat storage and thermochemical storage. Solar/thermal collectors are classified as low-temperature, medium-temperature or high-temperature, and the suitable heat transfer media is selected primarily based on the operating temperature.

Low-temperature collectors can make use of refrigerants/phase-changing materials, water, water-nano fluids and water-glycol mixtures as heat transfer media. Water-glycol mixtures and hydrocarbon oils are generally preferred for medium-temperature collectors. High-temperature systems make use of hydrocarbon oils, nanofluids and molten salts as heat transfer and storage media. Research is ongoing for different heat transfer media—such as air/compressed gases, molten metals, fluidized solid particles and ceramics—to achieve better properties to make them suitable for storage applications. The main challenges for energy storage are finding a commercially viable solution and committing to high investment costs.

Parabolic trough collectors

PTCs consist of solar collectors (mirrors), heat receivers and support structures (FIG. 5). The parabolic-shaped collector is fabricated by forming a reflective surface for concentrating sun rays to its focal line. The receiver consists of an absorber tube (usually metal) inside an evacuated glass envelope. The absorber tube is a coated stainless steel tube, with a spectrally selective coating that absorbs the solar (shortwave) irradiation well, but emits very little infrared (longwave) radiation. This helps reduce heat loss. Evacuated glass tubes are used because they contribute to reduced heat loss. PTCs are one of the most mature solar/thermal technologies and have been in commercial use for the last four decades. PTCs have an operating range of 290°C–550°C.

Fig. 5. Parabolic trough collector.

Fig. 5. Parabolic trough collector.

Linear fresnel reflectors

LFR systems (FIG. 6) generate a line focus onto a downward-facing receiver. The long row of plain or slightly curved mirrors is focusing onto the receivers. These systems are single-axis tracking, and the downward-facing cavity reduces convection heat losses. The cost of an LFR system is lower compared to a parabolic trough system due to the flat mirrors used and the lower requirements of the supporting structure, which is mounted closer to the ground. An LFR system has an operating range of 250°C–390°C.

Fig. 6. Linear fresnel reflector.

Fig. 6. Linear fresnel reflector.

Solar towers

STs, also known as central receiver systems (CRSs), use hundreds or thousands of small reflectors (called heliostats) to concentrate the sun’s rays on a central receiver placed atop a fixed tower (FIG. 7). The concentrated power of the tower concept achieves very high temperatures. The concept is highly flexible; designers can choose from a wide variety of heliostats, receivers and transfer fluids. STs have an operating range of 250°C–650°C, which can be extended to 1,000°C–1,200°C by using air/helium as the heat transfer medium. At the same time, STs are a more expensive option than the other technologies mentioned here, due to the complex receivers and high-temperature domains.

Fig. 7. Solar power tower.

Fig. 7. Solar power tower.

Non-concentrating collectors

These collectors (FIG. 8) are either flat-plate or evacuated tube collectors. Compared to concentrating collectors, non-concentrating collectors can only be used in low-temperature applications that are limited to 120°C. The cost is also low compared to concentrating collectors. The selection of suitable collectors for the application depends on the operating temperature of the application. Collectors and their corresponding temperature ranges are tabulated in TABLE 3.

Fig. 8. Non-concentrating collectors.

Fig. 8. Non-concentrating collectors.

CASE STUDIES

Case studies for different high-temperature requirements for various solar/thermal technologies are discussed in detail here. The absence of solar energy at night and during cloudy days can be compensated by energy storage and a hybrid system—a solar energy system with a backup heat source from a conventional source—that will ensure consistent energy supply. Storage is excluded from Case Studies 1, 2 and 3, while Case Study 4 is a hybrid system with storage.

Case 1: ST systems for high-temperature process heat applications

Fig. 9. High-temperature application schematic.

Fig. 9. High-temperature application schematic.

Traditionally, fuel oil, fuel gas and natural gas are used for heat requirements at high temperatures. Most reactions, such as steam reforming, catalytic reforming and gasification, require high temperatures in the range of 500°C–1,000°C, as well as high heat flux due to their endothermic nature. High heat flux and temperature requirements make the entire process highly energy intensive (FIG. 9).

Utilizing solar/thermal energy to supply heat to these reactions, directly or indirectly, will be a breakthrough in the integration of solar/thermal technology in oil and gas processing. Different solar thermal technologies can be considered as integration options, but the constraint comes with the operating temperature. Technologies with high concentration ratios, such as a central receiver system/dish system, offer an operating temperature of 600°C–1,000°C by using heat transfer fluid (HTF) media like air/helium. Molten salt as an HTF is a viable option for temperatures below 600°C.

A high-temperature application requires a temperature of 950°C, and this can be attained by using air as an HTF medium, along with  a central receiver system with volumetric air receivers. While this is not a mature technology, the prototypes developed have shown a positive output.

Heliostats concentrate the solar rays onto a volumetric receiver, which heats the air to the desired temperature. The high-temperature air exchanges heat in the heat exchanger/reactor, and the low-temperature air is returned to the central receiver system through a blower. The results of the case study are summarized in TABLE 4.

A 20 MW-t (megawatts thermal) capacity system was simulated to explore its technical viability. A design point DNI of 800 W/m2 was chosen, considering the hourly radiation profile available for western India. An inlet temperature of 450°C and an outlet temperature of 950°C were considered. Heliostats of standard size 12.2 m2 were used for the analysis. In the high-temperature segment, the payback period exceeds 10 yr due to thehigh capital expenditure (CAPEX) of the high-temperature ST system.

Typically, the central receiver system has an initial investment of $0.7 MM/MW, including the cost for heliostats, tower and receiver, and a land requirement of 2.1 acre/MW–3.5 acre/MW without the storage system. The addition of a thermal storage system to ensure a continuous energy supply requires three to four times the average footprint, as well as the same required CAPEX.

Case Study 2: Integration of solar/thermal crude heating application

Process heating is an important part of crude oil production. A significant amount of heat is required to preheat crude before it is processed in a crude distillation unit (CDU). Generally, refineries rely on fuel oil-fired heaters for preheating crude. Because the quantity of crude processed per day is large, so is the required amount of heat. In this case study, a typical plant processing crude with a heat requirement of approximately 38 MMKcal/hr was considered, with a refining capacity of 4.5 MMtpy.

This demand was supplied through fired heaters that use fuel oil to heat the crude to a temperature of 302°C–380°C. The conventional system was partially replaced by the proposed system with a capacity of 12 MW-t designed to heat the fluid from 302°C to 320°C. The proposed system used a PTC system for harnessing solar energy into process heat. The integration is illustrated in FIG. 10. Since the temperature requirement was in the range of 300°C–400°C, a PTC system or dish system was suitable for this application. The existing system was modified by incorporating a PTC system and integrating it with the existing system with a shell-and-tube heat exchanger. HTF was heated from a temperature of 340°C to 380°C in the PTC, and it was used to heat the crude oil in a heat exchanger by increasing the temperature of crude from 302°C to 320°C.

Fig. 10. Solar/thermal integrated crude heating.

Fig. 10. Solar/thermal integrated crude heating.

After exchanging heat, the HTF (Therminol) was again fed back into the PTC circuit. Crude was further heated from 320°C to 380°C in the fired heater itself, reducing the fired heater’s load. Typically, a PTC system has an energy collection of 930 MWh/yr/acre–1,150 MWh/yr/acre, and requires an investment of $300/MWh (TABLE 5).

The simulation provided an energy output of 18,156 MWh/yr, replacing 5% of the heat required for the heating of crude from 302°C to 320°C. The partial replacement of the fired heater for crude heating appears to be a viable option, as it replaces 5% of energy from solar with a 12-MW-t system. The project is providing a simple payback period of approximately 5 yr.

Case Study 3: Partial boiler replacement with LFR-based solar/thermal system

A refinery’s steam requirement plays a major role in its energy consumption. Low-pressure (3.5 barg/150°C), medium-pressure (10 barg/250°C) and high-pressure steam (35 barg/360°C) are used for different process requirements. Captive utility boilers are a source of steam and operate either on fuel oil or natural gas. The combustion of fuel oil or fuel gas also contributes to refinery emissions, as well as to the total specific energy consumption. As most of the captive boilers cater to refinery processes, the capacity of these systems is usually huge. Typically, a 1-MMtpy refinery requires 35 tph–80 tph of steam through its utility boilers. Replacing a part of the steam requirement with solar/thermal technologies can reduce CO2 emissions and save a significant amount of fossil fuel (FIG. 11).

Fig. 11. Solar/thermal steam production.

Fig. 11. Solar/thermal steam production.

This case study had a steam requirement of 15 tph at 10 bar, and utilized LFR solar/thermal technology. The results are summarized in TABLE 6.

The system is now providing an annual solar heat generation of 18,341 MWhr/t and reducing the CO2 emissions by 4,775 tpy, with a simple payback period of approximately 4 yr. Typically, an LFR system requires 0.43 acre/tph–0.6 acre/tph of steam production, while investment costs vary from $0.41 MM/tph–$0.62 MM/tph, and can reduce CO2 generation by 0.19 kg of CO2/kg of steam production.

Case Study 4: Solar/thermal-based vapor absorption refrigeration

In a typical petroleum refinery, approximately 12% of electricity is used for heating, ventilation and air conditioning (HVAC) and lighting, providing sufficient impetus to explore space cooling systems. Solar/thermal-based vapor absorption systems make use of heat generated from solar radiation to operate. A non-concentrating type of solar/thermal system is most suitable for the low-temperature application. The temperature needed for the input heat changes is dependent on the required cooling capacity. Water/steam can be used as the heat input media, and this provides flexibility to match the heat requirement with the cooling capacity. The solar-based vapor absorption refrigeration (VAR) system comprises a vapor absorption chiller, solar collectors, a cooling tower and heat storage.

The configuration of the integration of solar/thermal technology with a VAR system is illustrated in FIG. 12.

Fig. 12. Solar/thermal VAR system schematic.

Fig. 12. Solar/thermal VAR system schematic.

The case used for the analysis had a cooling requirement of 15 TR for the space cooling. Boiler and thermal storage were considered to compensate for the non-availability of solar hours and to ensure continuous supply throughout the year. The VAR system required an input of 71 kW-h to generate a cooling power of 53 kW. The system used hot water at 86°C to produce chilled water at 9°C (TABLE 7).

The chiller was integrated with solar/thermal storage of 400 kWh and a 115-kW boiler to ensure a continuous and reliable supply. The system generated heat of 201 MWh-t from solar collectors, while fossil heat generation produced 409 MWh-t. The total heat balance between solar and conventional sources is illustrated in FIG. 13. The system replaced 32.6% of the fossil heat to produce a cooling output of 53 kW, with a payback period of 4 yr.

Fig. 13. Annual energy balance, solar VAR system.

Fig. 13. Annual energy balance, solar VAR system.

This case study for a low-temperature application is the simplest use of solar heat in the low-temperature segment. Other potential uses include desalination, low-temperature steam production through solar/thermal systems for heat tracing, and storage tank heating. Oil and gas companies can perform various case studies by selecting suitable technology for their applications. Solar/thermal component costs are expected to drop, and the options presented in these case studies will continue to become more attractive, as seen in the case of solar photovoltaics (PV), where the power price has fallen from $0.17/kWh to $0.04/kWh in just a few years.

Challenges and limitations

The oil and gas sector faces various challenges and limitations while adopting and integrating renewable technologies. Reliability is the most prominent challenge, and the oil sector requires heat storage or a hybrid system while utilizing renewable energy. The high initial cost of the solar/thermal system creates budgetary concerns, but that cost is demand driven. The solar/thermal system offers 35%–40% efficiency in terms of conversion of solar energy into process heat, which is lower compared to fossil fuel heat conversion. Integration presents another challenge—e.g., steam/heat generation through a solar/thermal system must be integrated with the existing utility boiler and, in the case of the low flow of steam from the solar/thermal system, the utility boiler must ramp up before dropping the steam header pressure.

Maintenance of a solar/thermal system is significant, as the mirrors require regular cleaning to remove the dust and fine particles present in an open atmosphere. Improper cleaning mechanisms can badly affect the performance of the system. Perhaps the greatest challenge is convincing the oil and gas sector that the adoption of renewable energy is feasible, and can achieve the necessary efficiencies and return on investment (ROI).

The path forward

The first step toward effectively utilizing the available solar/thermal potential in the oil and gas sector is a combined evaluation by an oil and gas engineering consultant, the solar developer and the refinery client. The engineering consultant must be convinced of the benefits of adopting CST technologies, and then should serve as an advocate of those technologies. Using their experience in the oil and gas sector, this group will play a major role in the integration of these technologies and in the reduction of cost of these plants. The solar developer must develop methodologies that meet the requirements of the oil and gas industry. The close association of the solar developer, manufacturer/supplier, engineering consultant and client will lead to the development and improvement of these technologies.

The oil and gas industry must increase its awareness of the potential benefits of solar/thermal integration. This begins with the identification and classification of the temperature range and heat requirements for the installation of solar/thermal systems, including the need to review land availability, ownership, complexities in leasing procedures, etc.

Partial replacement of industrial heat is a feasible and achievable goal. The refinery should explore the carbon credit benefit from local government, as well as local policies that are favorable for the deployment of renewable energy technologies. Oil and gas companies can also include the integration of renewable energy in their social responsibilities within the category of corporate social responsibility (CSR). HP

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