It was mentioned in Chapter 1 how in recent years, the pollution regulations have led to an increase in the energy needs of a refinery. To cope with this increase, three solutions are available:
• the simplest way is to find alternatives to save the energy available in the refinery and not waste it;
• use less severe and unconventional processes;
• oil gasification and removal of CO2 produced in the refinery.
Since this paper is based on carbon dioxide capture, the first two points will be dealt with in this chapter. In the refinery there is a large use of electrical and thermal energy; in particular, thermal energy is much more used than electricity. The heat is produced either directly in the furnaces or is exploited by steam. It is convenient for a refinery to work in cogeneration to produce heat and electricity. The fuel used to produce steam and electricity is mainly the residue that remains from processes such as fuel oil, vacuum wastes, refinery gas and FCC coke. In general, most of the processes are energy-intensive, those that require more specific energy are the processes to produce lube-oils (used only in a few refineries), alkylation and isomerization. In Table 2.6 it is possible to see the specific consumption of thermal energy required per barrel processed. Speaking instead in absolute and non-specific terms, the processes that consume a lot of energy are those in which large feedstocks are treated such as atmospheric and vacuum distillation which consumes 35 to 45% of the total energy required by a refinery. This is because every barrel of oil that enters the refinery goes through this process. Another energy-consuming process is the hydrotreatment process.
Table 2.6: Energy use by refining process units (MJ/bbl).
Minimum Maximum Atmospheric distillation 90 200
Vacuum distillation 50 120
Visbreaking 100 150
Delayed coking 120 250
Fluidized catalytic cracking (FCC) 50 180
Hydrocracking (HCC) 170 340
Hydrotreating 60 180
Catalytic reforming 220 360
Alkylation by H2SO4 350 360
Chapter 2
Alkylation by HF 430 430
Etherification 310 600
Isomerization through isobutane 360 360 Isomerization through isopentane 100 250 Isomerization through isobutylene 480 480 Lube-oils production 1500 1500
The Figure 2.17 shows the estimated energy consumption for the major processes of the refinery. Remember that 1 btu equals 1055 J.
It is now interesting to see ways to save energy and be able to use it for other purposes which can be the capture of CO2.
As has been seen previously, the use of waste heat appears to be the most promising alternative in the short term while the implementation of new processes can be an alternative for the medium and long term. In refineries there are flows that have different temperatures, often even hundreds of degrees. One way is therefore to exchange the heat in a hot current
Figure 2.17: Estimated energy use by petroleum refining process. Energy use is expressed as primary energy consumption. Electricity is converted to fuel using 10,660 Btu/kWh (equivalent to an efficiency of 32% including
transmission and distribution losses). All steam is generated in boilers with an efficiency of 77%. [32]
with a cold current, saving the production of heat from an external source. These flows can be summarized as:
• use of waste heat in absorption refrigeration systems;
• use of waste heat to pre-heat feeds;
• heat and/or mass (water and hydrogen) integration using basically pinch techniques;
• improvement of furnaces efficiencies combined with computer-controlled combustion;
• direct feed of “intermediary products” to processes without cooling and storage, aiming at recovering the waste heat of these hot products. In other words, the heat contained in the distillation products can be used to heat another stream instead of ending up directly in the tank;
• use of heat pumps;
• decreased film temperature and increased turbulence on heat transfer surfaces;
• insulation of buildings and process units;
• adoption of steam management;
• Steam used for stripping and other processes is usually dispersed into the atmosphere.
In addition to not wasting the heat produced, the economic side also comes into play. In 2010, in the United States, 9 billion of dollars was spent on energy purposes by the oil industry sector. The graph in the Figure 2.18 shows the costs incurred annually in recent
Figure 2.18: Annual energy costs of petroleum refineries in the United States 1988-2010 for purchased fuels. [32]
Chapter 2
years to produce energy in the refinery sector. The graph does not consider the energy produced on site in the plant and excludes the value of fuels generated in the refinery and then used for this purpose; consequently, the cost is much higher.
The best technique among those listed is the use of pinch tecniques and can lead to a reduction of 10% to 20% of energy consumption. This technique is based on the optimization of heat exchangers, considering the enthalpy of the flows and their temperature.
Furthermore, another 2% reduction in consumption can be made by improving the recovery heat. Of course, it is not possible to use all the streams in heat exchangers because in many cases it is not convenient; the flow rates and costs are not sufficient to cover the savings given by the saved energy. Many streams have a lot of energy but are in difficult or inaccessible points, for example the gases leaving the FCC are difficult to exploit.
Atmospheric distillation can be a good choice to evaluate thermal integration with other processes because it has a large stream with a lot of energy.
Returning now to the analysis of the pinch point, that is the minimum temperature difference that can exist in a heat exchanger between the two streams, it is very affected by fouling. In many cases, fouling shifts this temperature difference from the canonical 10-20 °C to over 40 °C, affecting and worsening the efficiency of the exchanger. A first step is therefore to do maintenance often to avoid accidental risks and to remove the formation of fouling.
Another solution is to use advanced process controls that allow to improve process efficiency not only from an energy point of view but also from a production point of view. It translates into less down time, reducing maintenance costs. For example, it has been shown, in a specific case, that an FCC unit saves up to 0.05 $/bbl and the saved energy can reach 2-4%
of fuel consumption. One problem is the development of sensors capable of measuring information in very aggressive environments with high temperatures.
There is also the possibility of replacement of the conventional atmospheric and vacuum distillation units as they consume most of the heat produced (about 30% of the total). In the short term it is possible to use recovery heat and the possibility of integrating more distillation units. In the long term, however, it is possible to integrate the different distillation columns in a single reactor.
The use of vacuum pumps and surface condensers can replace barometric condensers in many refineries. Replacing the steam ejector by vacuum pump reduces the sour water flow and increase energy efficiency; in fact, the replacement of steam ejectors with vacuum pumps increases the electrical energy consumed but reduces the heat consumed, the cooling water consumption and consumption of agents used for conditioning cooling water. Taking a global balance, the energy saved is positive and therefore it is convenient.
Finally, there is the possibility of using pumps with variable speed as the pumps, in the refinery, often work in variable conditions. Normally the control is done with rolling valves and therefore wasting energy. Using frequency inverters for electric motors drives make possible to replace pumps that work at fixed rotation speeds. It is better to replace the pumps that operate with a high flow rate such as the pumps that bring the crude oil to the
atmospheric distillation column. Obviously, this is the last way to go as it is expensive and already with the alternatives described above it is possible to have excellent energy savings.
The Table 2.7 summarizes the possible alternatives
Table 2.7: Energy saving potential in an oil refinery.
Energy saving option Estimated fuel saving
potential (percentage of total fuel consumption)
Applicability
Heat integration and waste heat recovery
10% Fully applicable
Fouling mitigation 2% Fully applicable
Advanced process control 2% Fully applicable
Replacement of topping units by:
• Thermal cracking
• Progressive distillation
• Dividing-wall distillation
17%
15%
15%
Only for new refineries.
Pumps and advanced motors with variable speed
1% (Percentage of the electricity consumption).
Vacuum pumps and surface condensers
- Application restricted by
steam recovery and heat integration.
Methodology
In this chapter I will start by describing the works on the topic present in the literature because they represent a solid starting point. Subsequently, the Aspen Plus software will be presented, which will be used to carry out the calculations related to the thesis work.
Continuing in the paragraphs, you will find a part dedicated to the construction of two carbon capture models on Aspen Plus using the literature data because through them I will be able to validate the two models; these models will be validated in Chapter 4. The last paragraphs are dedicated to the sizing of the CO2 capture system starting from a flue gas flow. Finally, the state of the art of heat pumps is presented because an application will be studied to produce the heat necessary for the reboiler.