Low carbon emission oil refinery through post-combustion technology and its heat integration. Politecnico di Milano

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Politecnico di Milano

SCHOOL OF INDUSTRIAL AND INFORMATION ENGINEERING Master of Science – Energy Engineering

Low carbon emission oil refinery through post-combustion technology and its heat

integration

Supervisor

Prof. Davide BONALUMI

Candidate

Federico STANGONI – 913153

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Acknowledgements

È per me un piacere scrivere questa pagina perché è dedicata alle persone che hanno contribuito, aiutandomi e standomi vicino durante questo percorso.

Prima di tutti, voglio ringraziare il Prof. Davide Bonalumi, che in questi difficili mesi è riuscito a dedicarmi del tempo ed ha saputo guidarmi, con suggerimenti, nelle ricerche e nella stesura dell’elaborato.

Ringrazio la mia famiglia, perché a loro devo tutto. Ringrazio i miei genitori, Emma e Felice, per avermi sostenuto economicamento durante questi anni, senza di loro non sarei qui a scrivere queste righe. Ringrazio in particolare mia mamma, che mi ha sempre appoggiato nelle mie scelte e dato preziosi consigli. Ringrazio mio fratello, Fabiano, che è una spalla su cui contare sempre, che essendoci passato prima di me, ha saputo darmi consigli preziosi su come affrontare il mondo universitario e mi ha guidato a vivere in una nuova città come Milano per questi cinque anni. Ringrazio i nonni, che purtroppo non sono qui in questo momento ma sono sicuro che sarebbero stati orgogliosi di me. Ringrazio mia nonna Erminia che mi è stata sempre accanto durante questi anni e che non vede l’ora di comprarmi il vestito per la laurea.

Ringrazio Angela e Antonia, due persone che sono entrate nella mia vita tutto d’un tratto ma che anche grazie a loro non mi è mai mancato nulla. Le ringrazio anche per essersi sempre interessate a me e standomi vicino durante questi anni.

Un particolare ricordo a zia Maria, che se ne è andata da poco, chiedendomi fino all’ultimo quando mi sarei laureato.

Ringrazio la mia fidanzata Alessia che mi è sempre stata accanto durante l’ultimo anno e mezzo, facendomi forza e mai imponendosi sui miei impegni di studio.

Ringrazio i miei compagni di avventura, il gruppo del Tutorato, insieme a loro questi anni sono passati in fretta. Sono i migliori compagni di università che potevo trovare, sempre pronti a incoraggiarti e a sostenerti anche quando un esame è andato male, compagni di studi in biblioteca, compagni affidabili di progetti e di avventure.

Ringrazio gli amici di sempre, con loro ho passato molto tempo e li ho sempre sentiti vicino, anche quando non ci si vedeva perché dovevo studiare.

Ringrazio i miei colleghi arbitri, che sono stati e sono oltre che amici, una scuola di vita e di insegnamenti preziosi.

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Extended Abstract

Chapter 1: Introduction

Climate change is the topic of the day and a major cause is the production of carbon dioxide that comes from burning fossil fuels. An important slice of these emissions is attributable to the industrial sector. In fact, 33% of CO2 emissions come from the industrial sector. Of this slice, 7% is produced by refineries. There are not many works in the literature that deal with the reuse of waste heat to avoid additional carbon dioxide emissions. Furthermore, in most processes, the production of carbon dioxide is inevitable, and it becomes important to use techniques to capture the gas after combustion. Post-combustion capture is the most widespread technique now and is a minimally invasive technique in the case of a subsequent construction to the existing plants. This technique requires heat to regenerate the solvent and this is where the waste heat energy comes into play. A refinery has a large production of heat because a high temperature is required in most processes. It is estimated that 7 to 15% of the crude oil that is processed is used in the production of heat. Furthermore, in recent years, laws have imposed an increase in the quality of fuels, increasing the thermal demand in processes to obtain the specifications required by law. From the annual report of the British Petroleum oil company, it emerges that worldwide, in 2019, CO2 production increased by 0.5%. A study by the United States Environmental Protection Agency shows that the production of carbon dioxide from refineries has remained almost constant over the last decade. Another fact to consider are the future scenarios and "World Energy Outlook 2020"

presents four of them. The first two are the most realistic while the other two are optimistic.

The study shows that in the first two more realistic scenarios, the sources of energy most used in 2030 will remain almost constant, only coal will have a decrease in demand. In terms of fuel demand, oil will remain the industry leader until 2040 and consequently refineries will play a key role in the treatment of crude oil.

There are three techniques for capturing CO2 on a large scale: oxy-firing, pre-combustion and post-combustion.

The European scenario presents 89 refineries in 2020, of which 74 are active. Covid19 has had a big impact on the economy and the numerous lockdowns have led to a lower demand for fuel and therefore many refineries have stopped or reduced production. Taking advantage of the opportunity, some plants have undergone a restructuring and energy improvements, while others have started the conversion process towards the production of biofuels.

There are several techniques for using CO2. Transportation takes place via pipeline or tanker trucks. Carbon dioxide can be used in oil fields to maintain pressure, it can be injected into

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geological formations that are able to retain it, it can be injected into the sea floor so that it is dissolved in water or there are processes in which can be transformed, for example in the food industry or for the production of urea.

Chapter 2: Carbon Capture technologies and Oil refinery processes

The previously mentioned CO2 capture techniques will be analyzed with the related plant scheme in a refinery.

In the post-combustion carbon capture, the flue gases are cooled by water in direct contact before entering a compressor which has the purpose of overcoming the pressure losses in the absorber. When the flue gases enter the absorber, they meet a solvent which can be MEA so that up to 90% of the CO2 present in the stream is removed. The CO2 passes into the liquid solution and must be separated from the MEA. The carbon-rich solution reaches the stripper where the CO2 is separated from the amine and water solution and is ready to be transported.

In the cycle, a reintegration flow rate must be foreseen as the regeneration is not perfect and the MEA and water can pass into the gaseous phase and leave the system.

The pre-combustion capture process begins when a hydrocarbon feedstock is fed to an oxygen or air-blown pressurized gasifier or reformer, where it is converted to syngas. The syngas passes through a shift reactor which increases the CO2 and H2 content at high temperature and pressure.

The last capture process is the one called Oxyfuel combustion CO2 capture in which the fuel reacts with pure oxygen which is separated from the air by an air separation unit. This process is proposed for new plants as it is difficult to modify the existing layout.

The solvents used in post-combustion carbon capture are amines that can be classified into three types by their behavior. The most used solvents are MEA, DEA and piperazine.

A refinery is made up of many processes that transform crude oil into valuable products that can be sold on the market. The main processes will be described and analyzed. The first process is atmospheric distillation. Before arriving at the atmospheric distillation unit, the crude is treated to remove the salt and other contaminants present. The purpose of atmospheric distillation is to separate the crude oil into different components and then continue the treatments in subsequent processing units.

The fluid catalytic cracking produces distillates that feed the alkylation unit while the heavier compounds can be converted into diesel; the remaining part is considered fuel oil. Fluid catalytic cracking unit is a major source of CO2 emissions. Alkylation is a refinery process that transforms lighter products into valuable products. Lightweight products leaving the cracking unit cannot be sold on the market and therefore go to the Alkylation unit.

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Extended Abstract

Another conversion process is the isomerization of lighter compounds such as hexane and pentane. In fact, they find a conversion into high-octane molecules in the achylation and therefore need their own process.

Within the treating units, there are those processes that remove the impurities of the molecules. Hydrotreating processes have become increasingly important due to the continuous decrease of the sulfur concentration limits within the fuel.

Another very important process is catalytic reforming. It is a process that transforms heavy naphtha into high octane gasoline. In this process, hydrogen is produced which can be used in other processes such as hydrotreating and hydrocracking. Catalytic reforming is a key process in the production of gasoline components with a high-octane number.

In most refineries, hydrogen is produced on purpose as it is required in hydrotreating and hydrocracking processes. The reformer unit is unable to supply all the necessary hydrogen and therefore special processes are required. There are two processes in the refinery that have this purpose: steam reforming or through a partial oxidation of the fuel.

Chapter 3: Methodology

The chapter presents the works present in the literature that deal with the topic of CO2 capture in post-combustion using MEA as a solvent. Within this bibliographic review we will have on the one hand works that deal with the use of recovery heat to supply the reboiler of the stripper, while on the other hand there are works that propose changes to the standard capture system to improve the efficiency.

The Aspen Plus v11 software which will be used for the thesis work is then presented. In fact, two different models will be built on it and then validated by data present in the literature.

The conditions of the carbon capture system in the literature are presented, from the composition of the gases to be treated, to the characteristics of the solvent used. The dimensions of the absorber and stripper columns are also provided. It must be remembered that the property method is the method used for the presentation of thermodynamic and kinetic parameters for physico-chemical properties and inter-species interactions (reactions) of all species involved in the chemical process and for parameters that are not available in computer databases. In the following paragraph two different methods will be described to obtain the same result.

The first validated model is the ELECNRTL for the liquid phase; the ELECNRTL method calculated via non-ideal models is used for liquid phase material (such as, water, amine and hydramine) to absorb acid gas while for the calculation of the parameters of the gas phase, Redlich-Kwong equation is selected. This model is built at equilibrium. The equilibrium model, unlike the rate-based one, is much simpler to calculate and achieving convergence is faster however it has been corrected with the use of Murphree efficiency.

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The second model studied is the ENRTL-RK which is present in the Aspen Plus sample library. Consequently, the file will already present the chemical reactions that take place and the chemical components. In this simulation, the true components approach is used in fact in this case for the liquid phase the dissociation of the components in free and dissociated forms is considered.

The chapter continues with the presentation of a method for sizing the two columns. Two models were presented but now the idea is to use the best one for the sizing of a CO2 capture plant starting from a real gaseous stream arriving from a refinery.

The ENRTL-RK file will be used because a revision of the thermodynamic properties of the components is necessary to represent the process realistically with kinetics. The typical approach for sizing columns such as absorber is to use a series of sensitivity analyzes where the size of the column varies along with operating parameters over a given range of values.

It starts with the characterization of the flue gas stream and the solvent. The method consists in considering the columns alone and subsequently through the results obtained it is possible to couple the system.

The idea for sizing the height and diameter is to assume a column of infinite height and then derive the minimum solvent flow rate to obtain an efficiency of 90% and subsequently lower the height increase the solvent flow rate to obtain the same efficiency. For the calculation of the diameter, the automatic Aspen Plus calculator will be used once a flood base equal to 80% and a random base stage have been inserted, which will then be changed correctly by analyzing the temperature and steam flow profile along the column. I will stop with height sizing once the solvent flow rate becomes too high and consequently the regeneration costs will increase. As regards the stripper, the specifications to be considered for sizing are the composition leaving the bottom of the column which must be as similar as possible to the solvent entering the absorber. In fact, once the cycle is closed, the regenerated solution will enter the absorber.

The last part of the chapter is dedicated to the state of the art of heat pumps. A solution will be proposed to produce the necessary heat for the stripper by means of a heat pump.

Chapter 4: Case Study: Reference plant for the validation model

A refinery with a capacity of 100000 bbl/d will be considered in this work. The reference refinery is a hydro-skimming refinery, i.e. it is composed of primary distillation units, a gasoline block to meet the specifications of gasoline, a kerosene sweetening unit to produce jet fuel and middle-distillates hydrodesulphurization units for the production of diesel for land and marine propulsion and heating oil. There is no steam methane reformer in this refinery to produce hydrogen. Crude atmospheric distillation and vacuum distillation are not thermally integrated because they are built in different points of the area. Sea water is used as the condensation fluid, there are no cooiling towers. The layout of the refinery with the

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Extended Abstract

processes is then presented. The following processes are distinguished: crude distillation unit (CDU), saturated gas plant (SGP), LPG sweetening (LSW), kerosene sweetening (KSW), naphtha hydrotreater (NHT), naphtha splitter (NSU), isomerization unit (ISO), catalytic reformer (CRF), reformate splitter (RSU), kerosene hydrotreater (KHT), diesel hydro- desulphurisation unit (HDS), vacuum distillation unit (VDU), vacuum gasoil hydrotreater (VHT), visbreaker unit (VBU), amine regeneration unit (ARU), sour water stripper unit (SWS), sulfur recovery unit (SRU), waste water treatment (WWT), power plant (electricity and steam production, POW), utilities and off-sites unit.

The energy production section is then presented as it is the major source of carbon dioxide emissions and how a carbon dioxide capture unit may affect the refinery's emissions.

Consequently, the treatment of flue gases at the exit of the power plant will be studied.

In the final part of the chapter, the two models built in the Chapter 3 are validated using data available from the literature. The best model turns out to be the ENRTL-RK and will be considered as it has been developed with the rate-based model and consequently it is possible to build an optimization model for the dimensioning of the stripper and of the absorber.

Chapter 5: Results and discussion

In this chapter I will use the values calculated in Chapter 3 for the sizing of the absorber and stripper columns by going to see how the carbon dioxide emissions of the refinery vary. The data of the composition of the flue gases derive from a European research project of which Professor Bonalumi is a part and since it is still not completed, it is not possible to go into the details of the analyzed refinery. Consequently, I will limit myself to using only the composition of the flue gases of a chimney already described above and then apply the ENRTL-RK model in rate-based model present in the Aspen Plus template library.

Once the optimization of the two columns is finished, it is possible to merge them and build the complete process in Aspen. The carbon capture efficiency considering all the process is 92.1%. The CO2 capture plant can capture 7044.78 kg/h of carbon dioxide. Let's now see how it impacts on the global emissions of the refinery. Since the refinery is very large, and since there are many stacks, it is not possible to capture all the flows and it is also difficult to merge more than one. The chimney studied in this work is the A2-1 which corresponds to the emissions of atmospheric distillation. In fact, 33% of the total emissions derive from this process. With the use of carbon capture the carbon dioxide emissions of the refinery become equal to 16535 kg/h. 7060 kg/h of CO2 were removed which was equivalent to 30% of total emissions. The difficulty in capturing the remaining CO2 is due to costs that do not justify the feasibility of the capture plant. It could be interesting to take advantage of the proximity of some structures to convey more flow rates in a single system. Heat integration will then be evaluated through two ways: the first is through the use of a heat pump to produce the heat required by the reboiler, the second way is through the use of a natural gas boiler. The

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List of Figures

Figure 1.1: Greenhouse gases emissions from different sources in Europe in 2014. ... 1

Figure 1.2: GHG emissions by sector in Europe in 2017. [1] ... 2

Figure 1.3: Percentages of CO2 emissions of EU member states compared to 97.8 Mt/a (2018). ... 4

Figure 1.4: Emission trend of CO2 from the refineries sector from 2011 to 2019. [5] ... 5

Figure 1.5: Future energy park in 2030 [6] ... 6

Figure 1.6: Oil demand by sector in the Stated Policies Scenario, 2019-2030. [6] ... 7

Figure 1.7: Demand by fuel 2020-2040. [6] ... 7

Figure 1.8: Distribution of CCS project worldwide. [8] ... 8

Figure 1.9: Biofuels race: variation in the production of the top European companies in thousands of barrels of oil per day. [36] ... 9

Figure 1.10: Refinery sites in Europe. [11] ... 9

Figure 1.11: Process Flow Sheet patented by Saipem to produce urea. [17] ... 13

Figure 2.1: Post-combustion flow scheme. ... 17

Figure 2.2: Pre-combustion flow scheme. [25] ... 17

Figure 2.3: Oxyfuel flow scheme. [25] ... 18

Figure 2.4: Distribution of solvents used in 2300 patents. [8] ... 19

Figure 2.5: Structure and chemical formula of MEA. ... 19

Figure 2.6: Generalized flow chart of a typical refinery. [35] ... 21

Figure 2.7: Cut points and definition of distillate products as boiling ranges. [37] ... 22

Figure 2.8: Schematic diagram of the atmospheric distillation unit. [38] ... 23

Figure 2.9: FCC scheme process. [39] ... 24

Figure 2.10: The sulfuric acid alkylation process. [40] ... 26

Figure 2.11: Gas-phase equilibrium composition of hexanes at a pressure of 1 bar. ... 27

Figure 2.12: Isomerization using an aluminum chloride catalyst. [40] ... 28

Figure 2.13: Different configurations available for hydrotreating and hydrocracking processes. [20] ... 32

Figure 2.14: Change in the composition of the oil after the catalytic reforming process. [20] ... 32

Figure 2.15: Conversion of cyclohexane into benzene at different temperature and pressure. [20] 33 Figure 2.16: Catalytic reforming process. [41] ... 34

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] ... 37

Figure 2.18: Annual energy costs of petroleum refineries in the United States 1988-2010 for purchased fuels. [32] ... 38

Figure 3.1: General setup of the MEA/NH3 absorption processes. Equipment within dashed lines is only used in the NH3 process. [19] ... 42

Figure 3.2: Heat demands from the CCS capture plant before (Current situation) and after (Future situation) energy efficiency measures are being implemented. [19] ... 43

Figure 3.3: Available waste heat as a share of the heat demand for capture and CO2 capture rate with waste heat alone. [2] ... 44

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Figure 3.4: Process diagram of the industrial FCC unit (reference case of the paper "Study on the integration of fluid catalytic cracking unit in refinery with solvent-based carbon capture through

process simulation"). [21] ... 45

Figure 3.5: Process diagram of Case 2 of the paper "Study on the integration of fluid catalytic cracking unit in refinery with solvent-based carbon capture through process simulation". [21] ... 46

Figure 3.6: Chemical components participating in the CO2 capture plant. ... 53

Figure 3.7: CO2 capture process modeled in Aspen Plus. ... 54

Figure 3.8: Rate-based stage representation for the absorption process. [9] ... 55

Figure 3.9: Equilibrium reactions for the CO2-MEA-H2O system. ... 59

Figure 3.10: Aspen Plus process flowsheet for the CO2 absorption-stripping. ... 60

Figure 3.11: Changes to be made in the rate-base modeling window. ... 61

Figure 3.12: Infinite height absorber configuration. ... 64

Figure 3.13: Theoretical stage in which there is the highest flow of steam in the absorber. ... 65

Figure 3.14: Temperature profile in the 100 theoretical stages of the absorber. The maximum temperature value corresponds to the tray with the greatest steam flow. ... 67

Figure 3.15: Results of the sensitivity analysis. ... 68

Figure 3.16: Theoretical stage in which the largest diameter occurs. ... 68

Figure 3.17: Temperature profile along the absorber for different heights. ... 69

Figure 3.18: Partial flowsheet of the regeneration section. ... 72

Figure 3.19: Temperature profiles inside the stripper as the height varies. ... 76

Figure 3.20: Comparison of Steam-Turbine and Heat-Pump Operating Principles. [42] ... 77

Figure 3.21: Principle structure of a heat pump. [42] ... 78

Figure 3.22: Thermodynamic cycle for refrigerant fluid R407c. ... 79

Figure 3.23: Cycles for two refrigerants at the same operating temperatures and pressures. ... 80

Figure 3.24: Flowsheet of a cascade heat pump with a multi-stage R-718 cycle for steam generation or closed loop heat supply at different temperature levels. ... 83

Figure 3.25: Flowsheet of reversed Brayton cycle. ... 84

Figure 4.1: Simplified block diagram for reference case studied in the validation process. [33] .... 87

Figure 4.2: Refinery layout. [33] ... 89

Figure 4.3: Simplified Power Plant configuration considered in the layout plant model. [33] ... 90

Figure 4.4: Process flow diagram of the MEA process for post-combustion CO2 capture. [33] ... 96

Figure 5.1: Flowsheet of the plant on Aspen Plus. ... 102

Figure 5.2: Percentage of CO2 emissions out of the total. Based on mass flow rate. ... 104

Figure 5.3: CO2 emissions in kg/h before and after using the CCS in the refinery. ... 104

Figure 5.4: Heat pump cycle built in Aspen Plus. ... 107

Figure 5.5: Result of the sensitivity analysis to calculate the flow rate of the work cycle which must be able to heat the solution to be regenerated. Richtemp is the temperature of the solvent at exit of the reboiler. ... 107

Figure 5.6: Flowsheet for the simulation of a natural gas boiler in Aspen Plus. ... 110

Figure 5.7: Sensitivity analysis for the calculation of the methane flow rate. The word Temp represents the outlet temperature of the solution to be regenerated by the reboiler, Heat represents the heat exchanged and Gastemp is the temperature of the burned gases. ... 110

Figure 5.8: CO2 emissions in the three possible scenarios: without the use of CCS, use of CCS with heat pump and use of CCS with natural gas boiler. ... 113

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List of Tables

Table 1.1: Top five increases and decreases in oil consumption and production. ... 5

Table 1.2: CO2 emission sources in a typical refinery. ... 10

Table 2.1: Major CO2 emission sources at a typical refinery complex with available carbon capture technology. ... 16

Table 2.2: Technical advantages and barriers for post-combustion solvents. ... 20

Table 2.3: Summary of Catalytic Cracking Processes. ... 24

Table 2.4: Hydrogen consumption during hydrotreating for various feedstocks. ... 29

Table 2.5: Summary of the operating condition of hydrocracking processes. ... 31

Table 2.6: Energy use by refining process units (MJ/bbl). ... 36

Table 2.7: Energy saving potential in an oil refinery. ... 40

Table 3.1: How much CO2 can be captured using only excess heat, and how much that will be emitted to the atmosphere (rem=remain) or to be captured using primary heat. ... 43

Table 3.2: Summary of the results of the three case studied in the paper "Study on the integration of fluid catalytic cracking unit in refinery with solvent-based carbon capture through process simulation". ... 47

Table 3.3: CAPEX and OPEX cost breakdown for the anylys of the paper “Techno-economic analysis of excess heat driven post-combustion CCS at an oil refinery”. ... 48

Table 3.4: Overview of options for process modifications. ... 50

Table 3.5: Overview of process modelling results for the process modifications considered in the study. A negative saving indicates an increase over the reference case in the paper “Analysis of combined process flow sheet modifications for energy efficient CO2 capture from flue gases using chemical absorption”. ... 51

Table 3.6: Characteristics and composition of flue gases of the validation models. ... 57

Table 3.7: Characteristics and composition of the solvent coming from literature. ... 57

Table 3.8: Operating conditions of the absorber for the validation models. ... 57

Table 3.9: Operating conditions of the stripper for the validation models. ... 58

Table 3.10: Characteristics of the flue gases of the real case. ... 62

Table 3.11: Characteristics of the solvent. ... 62

Table 3.12: Simulation results where the solvent has the same flow rate as the flue gases. ... 66

Table 3.13: Results of the second simulation with a solvent flow rate equal to 80000 kg/h. ... 67

Table 3.14: Values obtained by varying the absorber height and solvent flow rate to obtain a capture efficiency of 90%. ... 69

Table 3.15: Results obtained for heights between 15 and 10 meters. ... 70

Table 3.16: Apparent flow rates obtained for an absorber height of 15 meters. ... 70

Table 3.17: Characteristics of the solvent to be regenerated obtained from the absorber modeling.71 Table 3.18: Construction features of the stripper for the first iteration. ... 72

Table 3.19: Flow rates obtained from the first iteration of the stripper at a height of 30 meters. .... 73

Table 3.20: Apparent flow rate and apparent mass fraction of the elements present in the stripper section. ... 73

Table 3.21: Specifications to be respected for the regenerated flow out of the reboiler. ... 74 Table 3.22: Variation of the apparent mass fractions as the stripper distillate rate varies for a height

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Table 3.23: Simulation results for a distillate rate of 14500 kg/h and a height of 30 meters. ... 75

Table 3.24: Results obtained by varying the height of the stripper and the distillate rate to obtain the desired composition at the outlet. ... 76

Table 3.25: Main heat pumps with a temperature above 90 °C. ... 82

Table 4.1: Properties of crude oils processed in the refinery. ... 85

Table 4.2: Overall material balance. ... 86

Table 4.3: Process units operating and design capacity. ... 88

Table 4.4: Refinery base loads of power and steam. ... 91

Table 4.5: Specific utility consumption for main process units. ... 91

Table 4.6: CO2 emissions per unit for the reference case. ... 94

Table 4.7: Summary of the characteristics of the absorber section. ... 95

Table 4.8: Summary of the characteristics of the stipper section. ... 95

Table 4.9: Comparison between the literature values and those calculated in Aspen Plus. ... 97

Table 4.10: Difference between the literature values and those calculated by the two models. ... 98

Table 4.11: Relative errors with respect to the literature. ... 98

Table 5.1: Design parameters optimized for the two columns in the real case simulation. ... 101

Table 5.2: Apparent flow rates coming from the operation of the plant in optimized conditions. . 102

Table 5.3: Flow rates of make-up and purge calculated through the Aspen Plus calculator. ... 103

Table 5.4: CO2 emissions from the main stacks of the refinery. ... 103

Table 5.5: Characteristics of the solvent to be regenerated. ... 105

Table 5.6: Characteristics of the stripper. ... 106

Table 5.7: Operating points of the refrigeration cycle for the R245fa fluid. ... 106

Table 5.8: Data used for the simulation in Aspen Plus in the working cycle of the heat pump. .... 107

Table 5.9: Simulation results for a flow rate of R245fa equal to 400000 kg/h. ... 108

Table 5.10: Consumption associated with compressor operation. ... 108

Table 5.11: Operating costs of the heat pump. ... 109

Table 5.12: Operating costs of the natural gas boiler with the assumptions described. ... 111

Table 5.13: Operating costs incurred in the case of the use of a boiler and a heat pump. ... 112

Table 5.14: Carbon dioxide emissions per hour of the two solutions proposed solutions. ... 112

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Introduction

1.1 Climate change

Climate change, in our day, is an ever-present and important topic. One of the main causes is certainly the increase in the production of carbon dioxide which, ending up in the atmosphere, increasing the concentration more and more and in this way the heat does not leave the earth and the temperature increases. This increase in temperature damages the planet and we need to look for technologies that replace, improve efficiency or new techniques to do what we already do today but avoiding the production of greenhouse gases.

A very important slice of greenhouse gas production in Europe is attributable to the industrial sector. Indeed, Figure 1.1 shows that 33% of greenhouse gas emissions derive from the industrial sector. One third of total emissions is attributable to the industrial sector. Although greenhouse gases are not composed solely of CO2, CO2 is one of the main ones; it is one of the major gases present in the atmosphere and occupies a portion equal to 10-20%.

Figure 1.1: Greenhouse gases emissions from different sources in Paper mills

1%

Chemicals

3% Glass works

2%

Cement works 8%

Other 4%

Steelworks 8%

Refineries 7%

Energy and heating

67%

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It must also be remembered that CO2 is present in the atmosphere also for natural causes. In recent years, however, man's anthropogenic activity has meant that the concentration of CO2

has increased considerably, and governments have had to intervene with laws and new taxes to avoid its uncontrolled production.

We have seen how much the industrial sector impacts compared to the energy production sector. Now from Figure 1.2 you can see the impact of greenhouse gas production from each sector in 2017. It is immediately evident that 17% derives from the industrial sector, if we add 18% of transport to this, it becomes a large slice that derives from the combustion of fossil fuels such as oil.

In this work, I will study the refinery and its impact on CO2 production, which, according to the article "Overview of the refining industry in the European Union Emissions Trading System (EU ETS)", is responsible for 7% of emissions of greenhouse gases in Europe. It is therefore important to try to limit this quantity. Furthermore, the refinery sector is chosen as a starting point as there are few studies regarding this topic as the available waste heat is almost absent and therefore new techniques to reduce CO2 emissions are still being studied.

It will be explained in the following chapters that the most convenient technique for limiting CO2 emissions is post-combustion as many European refineries have been working for years and it would not make sense to change the process upstream. Post-combustion is a minimally invasive technique from the plant engineering point of view, in fact it only requires the presence of space to build the suitable equipment.

Figure 1.2: GHG emissions by sector in Europe in 2017. [1]

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Chapter 1

The CO2 capture process, as will be seen below, requires the need to produce heat. This heat can come from hot streams or it can be produced on purpose. In the second hypothesis, further CO2 will be produced and therefore it is more convenient to use waste heat that comes from processes that would otherwise be lost. In the paper "Process evaluation of CO2 capture in three industrial case studies" three types of industries are compared: a kraft pulp mill, an oil refinery and an aluminum mill. In this article, available waste heat is calculated as the heat that can be extracted from flue gas. What emerges is that in the aluminum plant the recovery heat covers 60% of the heat necessary to have a CO2 capture efficiency of 85%; in the case of the pulp mill there is not enough heat available (low quality heat) while in the case of the refinery, the waste heat covers 25% of the total demand. This fact makes us think:

the heat available in the refineries is scarce as the processes have an excellent synergy and the heat is recovered to have very high process efficiencies. The same article also analyzes a new generation power plant where the available heat is almost infinite as there will be spills from the turbine which, however, will affect the production of mechanical energy.

Furthermore, the quality of the recovery heat in this case is very high even if we cannot properly speak of recovery heat. The boiler does not have to produce extra heat, it is the turbine that will see less steam flow. In this case, the heat is sufficient to cover the heat demand of the CO2 capture process even if part of the work produced will be renounced.

Quoting the article "Fuel specification, energy consumption and CO2 emission in oil refineries" it can be estimated that in a refinery from 7 to 15% of the crude oil input is used to supply heat to the necessary refinery processes. Over the years, however, this situation has changed, in fact the environmental laws that impose a better quality of oil products and shift towards low-grade crude oils in the world refining industry have meant that the thermal demand increases more and more. The increase in the quality of petroleum products results in low sulfur contents which normally require more energy. Consequently, to produce a cleaner diesel and gasoline, more energy is required and therefore more emissions of carbon dioxide which is a greenhouse gas as seen above.

Therefore, new roads are born that the refinery must follow in the medium term: alternatives are needed to avoid wasting the energy produced or the implementation of new processes that replace the existing ones. In the 27 European countries, 97.8 Mt of carbon dioxide were emitted in 2018 for fuel combustion in refinery. Figure 1.3 shows how these emissions are divided between the various European states.

This study is keen to point out that this sector is constantly expanding and will continue to expand in the coming years. Indeed, it emerges from numerous articles where many studies of future scenarios still hypothesize the presence in large quantities of oil products in energy production and transport.

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From the annual report "BP Statistical Review of World Energy 2020" prepared each year by the British Petroleum oil company, it emerges that, although in many countries CO2

production has decreased, in many others it has increased, bringing to the annual total (2019) an increase of 0.5% compared to 2018 and increased 1.1% compared to the last decade (2008-2018).

Again, in accordance with BP's annual report, CO2 emissions from electricity generation increased by 0.5% as well as oil demand increased by 0.9% compared to 2019. Even though refinery utilization dropped dramatically by 1.2%, daily production still increased leading to little or no change in oil demand. In Table 1.1 you can see how the demand and production of the main countries has varied.

Figure 1.4 shows the trend of recent years in the production of CO2 by the American oil industry, the image is taken from the United States Environmental Protection Agency. It is immediately evident how the production of CO2 from refineries has remained almost constant for many years; it is therefore interesting to study ways to break this trend.

Figure 1.3: Percentages of CO2 emissions of EU member states compared to 97.8 Mt/a (2018).

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Chapter 1

Table 1.1: Top five increases and decreases in oil consumption and production.

Also citing "World Energy Outlook 2020", four future scenarios are described taking into consideration the covid19 pandemic that has affected the whole world and all sectors, in fact the refinery industry has suffered as well as the price of oil leading to an economic crisis.

The first two scenarios are the most realistic while the last two are the most optimistic. These scenarios are:

Oil consumption Annual change (thousand b/d) Oil production Annual change (thousand b/d)

Increases Increases

China 681 US 1685

Iran 183 Brazil 198

India 159 Canada 150

Algeria 37 Iraq 148

Russia 35 Australia 135

Decreases Decreases

Mexico -88 Iran -1266

Italy -59 Venezuela -556

Pakistan -52 Saudi Arabia -429

Taiwan -52 Mexico -150

Venezuela -47 Norway -115

Figure 1.4: Emission trend of CO2 from the refineries sector from 2011 to 2019. [5]

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• Stated Policies Scenario (STEPS): is based on current policy and on the basis that the covid19 pandemic is controlled by 2021. GDP returns to pre-crisis value in 2021 while the global energy demand comes back in 2023.

• Delayed Recovery Scenario (DRS): this scenario is almost like the previous one but increases the damage caused by the pandemic. In this future, the global GDP is not recovered until 2023 while the demand for energy returns the same only in 2025.

• Sustainable Development Scenario (SDS): it is a scenario in which short-term investments are required to reduce emissions by more than 10 Gt in 2030 compared to the STEPS scenario. It is a very ambitious scenario as it requires a major transformation of the energy production sector.

• Net Zero Emissions by 2050 (NZE2050): it is a utopian scenario, which is used to see how it is possible to reduce emissions to 0 in 2050. In this scenario, the measures to be taken to have a future with zero emissions by 2050 are explored. A great acceleration is needed to reach this goal in the development of new renewable energy sources.

Figure 1.5 shows the scenarios with the relative composition of the future energy park in 2030 for the different scenarios. It can be seen how the demand for oil will increase in the case of the STEPS scenario while in the DRS scenario the demand will remain unchanged.

Only coal will go down.

Furthermore, the trend in fuel demand is also analyzed in the scenarios. Oil will always be the most used fuel until 2040 in the STEPS scenario; in the SDS scenario, the demand for fuel deriving from oil decreases even if it remains high. Figure 1.6 shows the request for oil fuel for the most current scenario (STEPS).

According to Figure 1.7 the demand for fuel will increase until 2040. The fact that the demand for fuel oil will increase is not linked to CO2 emissions in the refinery, however it is understood that crude oil must be processed before reaching the market. Processing takes

Figure 1.5: Future energy park in 2030 [6]

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Chapter 1

place in refineries and by increasing demand, it will also result in an increase in the current production capacity of the refineries. The increase in demand for crude oil distillates suggests how important the role of the refinery is. It was also seen in Figure 1.5 that even on the energy production side, oil will always be present, forcing refineries to work and occupy an important slice of the industrial sector for the next years

1.2 Carbon capture technologies

With the aim of significantly lowering CO2 emissions, technologies called carbon capture and storage were born. Improvements in terms of energy efficiency in a refinery will never completely remove CO2 production as it will continue to consume large amounts of energy and therefore CO2 emissions will remain high. One way to greatly reduce these emissions is

Figure 1.7: Demand by fuel 2020-2040. [6]

Figure 1.6: Oil demand by sector in the Stated Policies Scenario, 2019-2030. [6]

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• Oxy-firing: pure oxygen (O2) is used instead of air in order to obtain gases that contain only carbon dioxide and water. The main problem is the necessity to have a continuous oxygen supply that means high costs.

• Pre-combustion capture: first the syngas (CO + H2) is formed in the combustion chamber thanks to a partial oxidation of the fuel which is usually coal. The process takes place in a gasifier or, in the case of natural gas, using the water gas shift reaction. The syngas passes into the water gas shift reactors and what is obtained is CO2 and H2. Now the CO2 concentration is high and can be easily separated by physical absorption. Due to the high cost, this technique is not diffused.

• Post-combustion capture (PCC): CO2 is captured after the combustion process of the fuel. Different separation techniques are available: adsorption, physical absorption, cryogenic separation, membrane absorption or algal systems. The most used approach is through absorption with a chemical solvent as the concentration of CO2

is low (low CO2 partial pressure) due to N2 present in large quantities in the air. The great advantage of this technology is that it can be easily implemented in old refineries.

1.3 Refineries in Europe

In 2020 there were 89 refineries in Europe, of which 74 process crude oil. A total of 12660 kbbl/d are processed each day. There are currently 10 refineries in Italy that process crude oil for 11.2% of the European total. In 2019 there were 94 refineries of which 79 were operating. The covid19 epidemic shock has brought oil prices to a low price following numerous lockdowns around the world that have brought demand for oil to a minimum. As a result, refineries throughout 2020 suffered from this, leading to the closure or shutdown of many plants. Many plants have decreased production, others have closed and still others have used the opportunity to shut down the plant and begin the conversion to the biofuel sector. It must also be considered that the closure of a refinery is an expensive process

Figure 1.8: Distribution of CCS project worldwide. [8]

Pre- combustion

40%

Oxy-fuels 6%

Post- combustion

54%

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Chapter 1

because important equipment must be dismantled and above all it is necessary to restore the surrounding environment. Therefore, the companies that operate the refineries exploit alternative routes such as converting them into import terminals, using them for other industrial purposes or in many cases converting the refinery into a plant that produces cleaner biofuels by treating vegetable oils and waste oils. In fact, as can be seen in Figure 1.9, the main European companies have started the conversion and by 2030 they aim to produce many biofuels.

Figure 1.9: Biofuels race: variation in the production of the top European companies in thousands of barrels of oil per day. [36]

8

55 22

100 10

100 6

13

0 20 40 60 80 100 120

2020 2030 2020 2030 2020 2030 2020 2030

EniBPTotalRepsol

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The refinery in Europe is often associated and discussed for a low-carbon future. However, it also has many positive aspects ranging from providing jobs for many people, to manufacturing chemicals, to innovation and even economically. In Europe, about 100,000 highly skilled engineers, scientis and trade workers work in the refining sector with another 50,000 people employed in marketing and supply. Furthermore, on the economic side, the refineries bring 240 billion euros in taxes into the pockets of the states of the European Union, which correspond to 7% of revenues in Europe. From the refinery industry derive many raw materials and many indispensable products for our daily life such as bitumen, lubricants, waxes, fuels and chemical feedstock for sectors like transport and heating. For example, 50% of medical equipment is made with plastics derived from refining, the lubricant for over 40,000 wind turbines derives from crude oil and many billions are invested every year in research and development sector. The Table 1.2 shows the typical emissions of a refinery which will be justified in the following chapters.

Table 1.2: CO2 emission sources in a typical refinery.

1.4 What to do with CO

2

?

The question that arises is what to do with the captured CO2. With the evolution and expansion of CO2 capture processes, the need to give a second life to carbon dioxide arises.

Several ideas are reported and described.

Emission source Description Share of CO2

emission (%)

CO2 concentration (vol.) in the off-gas flow (%)

Process furnaces Heat generation via combustion of fossil energy carriers for distillation columns and reactors

30-60 8-10

Steam generators Process steam generation via combustion of fossil energy carriers

20-50 4-15

Catalytic crackers

Burn-up of petroleum coke 20-50 10-20

Hydrogen production

Reforming of hydrocarbons to H2 and CO2

5-20 20-99

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Chapter 1

1.4.1

CO

2

transport

First, it is important to study the transport of CO2. There are various ways. There are tanker trucks that can carry tens of tons of CO2. However, being a gas and being produced in large quantities, a pipeline is preferable while in some conditions it is also possible to transport by ship.

There are over 6000 km of CO2 pipelines installed in the United States to support enhanced oil recovery operations. To ensure that the fluid is maintained in a single phase, the duct must be kept at a pressure higher than the critical one which is for CO2 73.9 bar. Water is removed to prevent corrosion inside the pipes.

Let's now analyze the costs to compare the different transport technologies. As a base case we consider a pipeline that transports 10 MtCO2/year per year which corresponds to a cost equal to 1 $/tCO2/100 km. Using a truck, on the other hand, costs 7 $/tCO2/100 km. The ship, like the truck, has higher costs that are linked to the conditions of transport of CO2 that are −20°C and 20 bar. For intermediate transport, a pipeline is worthwhile while if you go over long distances, a ship can be competitive.

Transporting CO2 is however a very dangerous process as it is a gas heavier than air and accumulates on the ground. CO2, if inhaled in concentrations above 17%, can also lead to death.

1.4.2

Geological storage

In oil wells, daily production decreases over time as there is a decrease in pressure in the reservoir; moreover a large part of oil remains trapped in the pores of the rocks. The so- called EOR (Enhanced Oil Recovery) techniques are then used to increase the daily production. One of these techniques is the injection of CO2 into the reservoir with the aim of maintaining the pressure and releasing the oil from the rocky pores. It is important to underline that this is not a solution to fight climate change, but it is a way to use CO2 and make CCS profitable. Furthermore, these EOR projects help to spread pipelines for the transport of CO2. It has been proven by EOR studies that injecting CO2 into these rock formations is safe.

1.4.3

Target formation

In order to use a geological formation to store CO2, four criteria must be met.

1. The geological formation must have good permeability to let the water flow (for example bucket of sand).

2. The geological formation must be at least 800 m deep to allow the CO2 to remain in liquid form (the pressure must be above the critical pressure of CO2). In fact, at a depth of 800 meters, the hydrostatic pressure is about 80 bar which is greater than 73,9 bar.

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3. The target formation must have an impermeable caprock to ensure that the CO2

remains trapped.

4. It is preferable to have a large and thick geological formation so that large quantities of CO2 can be stored there.

In practice, the formations that have these requirements can be two: the oil and gas fields as seen above or deep saline formations

1.4.4

Ocean storage

It is no longer used today but has been studied in the past. The aim is to put the CO2 at 800 meters in order to dissolve the CO2 in the water. The main problem remains the fact that CO2

does not remain in the seabed but interacts with the atmosphere. A major problem is related to the marine ecosystem that is being altered.

1.4.5

CO

2

utilization

Today, instead of storing CO2, we try to find ways to use it for useful purposes. This technique is called carbon capture and utilization (CCU). Unfortunately, this path is followed by only 1% of the total CO2 produced per year. CO2 is used in some fields such as carbonating beverages, flash freezing of foods (beverage industry), and as an expellant in fire extinguishers. This is since transporting and storing CO2 is expensive in small volumes.

A more commonly used way is to transform CO2 into a fuel. CO2 is transformed into fuel using renewable energies as the aim is precisely to avoid the production of new CO2 for combustion.

In Canada there is a CO2 capture process capable of capturing 30 tons per day and the captured CO2 is then reused to encourage the growth of plant species in greenhouses.

Furthermore, the company Saipem, leader in the oil and gas sector and in the CO2 capture technology sector, has patented a highly efficient and sustainable plant to produce urea starting from NH3 and CO2. The process is illustrated in the Figure 1.11.

This process requires compressors to increase the pressure of ammonia and carbon dioxide, then there is a reactor where urea is formed and a stripper to remove vapors such as ammonia and carbon dioxide that is not converted to urea from the stream. There is also the carbamate condenser which condenses these vapors and the ejector which recirculates ammonium carbamate solution to the reactor. The pressures are around 150 bar and the temperatures vary from 155 to 205 for the outgoing flows. The NH3/CO2 reactor inlet ratio is 3.2-3.4 molar and is also used at low watercarbon dioxide ratio (0.4-0.6 molar). The reactor contains many plates of simple design and guarantees a conversion of the CO2 input of 62-64%. In the HP section, there is a total CO2 conversion of 85-90% considering the loop. The steam consumed in the stripper is almost totally recovered in the condenser. Despite the severe working conditions, the equipment lasts for more than 20 years.

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Chapter 1

Another idea of using CO2 is studied by Fernández-Dacosta and consists in the use of carbon dioxide in the synthesis of polyethercarbonate polyol. From his work it emerges that up to 16% can be saved compared to the conventional case, in fact CO2-based polyols cost around 1200 €/t which is 16% less than the cost of manufacturing conventional polyol. However, the demand for polyol is not sufficient to be able to use all the CO2 produced in Europe, in fact 10% of CO2 emitted would be enough for the European need for polyol.

1.5 Thesis objective

The objective of this thesis work is to start from the description of the processes present in the refinery and to develop two models in Aspen Plus v11 of a post-combustion process for the capture of CO2 starting from a stream with a reference composition. The stream used is taken from the literature and will show that the model built in Aspen Plus is really working, reflecting the data of the literature. The reference refinery is taken from a project carried out by SINTEF, one of Europe's largest independent research organizations.

After validating the two models, one will be chosen based on the design needs of the system.

Starting from this model, I will take a flue gas flow that derives from a real refinery and I will go to size the plant. A method for sizing the diameter and height of the absorber and stripper columns will be proposed. The heat needed by the reboiler to regenerate the solution containing the solvent will also be calculated.

The work is divided into the following chapters:

• Chapter 1 Introduction: description of the current scenario, with a brief outline of what will be analyzed.

• Chapter 2 Carbon Capture technology: the chemical processes of a refinery will be studied and the CO2 capture processes present on the market will be described in detail.

• Chapter 3 Methodology: the chapter begins with the description of the

Figure 1.11: Process Flow Sheet patented by Saipem to produce urea. [17]

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the description of the procedure carried out to build the two proposed models for the validation. Furthermore, it will be described how to size the absorber and the stripper through a method proposed in the literature. The state of the art of heat pumps will be presented.

• Chapter 4 Case Study: Reference plant: a detailed description of the reference refinery will be provided, with the characterization of the plant processes. It also describes the validation process performed on the models built in Aspen Plus.

• Chapter 5 Results and discussion: after dimensioning the system, the results shown on emissions are shown in this chapter. The impact of the adoption of a carbon capture on the global emissions of the refinery will be assessed. Two possibilities are presented to produce the necessary heat for the reboiler.

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Carbon Capture technologies and Oil refinery processes

The industrial sector produces one fifth of the total CO2 emissions, of this sector the refineries are the fourth for emissions. There are different emission points because the production of heat is necessary inside the refinery. This heat is produced using various fuels present on site. CO2 emissions can be divided into 4 branches: process heaters (30-60%), fluid catalytic cracking (20-50%), hydrogen production (5-20%) and utilities (20-50%).

Table 2.1 analyzes the processes listed above with the possible implementation of a CO2

capture method.

The three techniques previously mentioned CO2 capture techniques will be analyzed with the related plant scheme in a refinery.

2.1 Post-combustion CO

2

capture

The flue gases are cooled by water in direct contact before entering in a compressor which has the purpose of overcoming the pressure losses in the absorber. When the flue gases enter the absorber, they meet a solvent which can be MEA so that up to 90% of the CO2 present in the stream is removed. The gaseous flow rises to the top of the absorber and subsequently exits from a chimney and then disperses into the atmosphere. The solvent is enriched with CO2, then passes into a heat exchanger which recovers the heat of the lean amine stream from the reboiler of the stripper. The solution reaches the stripper where the CO2 is separated from the amine and water and is ready to be transported or stocked. In the case of streams with a high sulfur content, a desulfurization unit must be added to the process shown in Figure 2.1. Post-combustion is a very simple process that can be used in all types of combustion. To limit energy consumption, the CO2 capture unit must be close to the CO2

production sources such as FCC, boiler and hydrogen production units. Obviously, MEA and water will have to be added with a make-up flow as the regeneration is not perfect but there will be a part of water and MEA leaving the system.

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Table 2.1: Major CO2 emission sources at a typical refinery complex with available carbon capture technology.

Refinery process Description Capture methods and conditions

Post-combustion Pre-combustion Oxyfuel-combustion

Hydrogen production – Steam reforming (SMR)

SMR is a major single- point source of CO2

emission.

It can be applied with a 90%

capture of CO2.

SMR can be modified for pre- combustion carbon capture. By this route only 50% of CO2 can be captured.

FCC High CO2 emissions. It is important to remove sulfur due to the presence in the stream.

High energy consumption for post-combustion case

This technology can only be applied to new FCC systems and when large on-site heaters and boilers are replaced. Lower energy consumption compared to the post- combustion case.

Process heat (furnaces, boilers)

CO2 emissions are related to the combustion process of a fuel. (In previous cases, the emissions were due to process waste).

It is the simplest way to capture CO2.

Applying this technique involves re-bonding the flue gases to a cooling zone and a sulfur removal zone. It is also necessary to minimize the distances to reduce pressure losses.

When a processing unit contains several small eaters fired on refinery fuel gas in a very congested plot.

It can only be used when the heaters are running on the same fuel.

If the heaters can be sealed against air inlet and unit air separation added to the site, oxyfuel might be a good option.

Difficult to apply to existing plants.

Utilities (production of electricity and steam)

Post-combustion and oxy-fuel are the most promising way to capture CO2.

It can be used on any flue gas stream.

It can be used in those cases where the system allows to seal the inlets to the outside air.

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2.2 Pre-combustion CO

2

capture

A hydrocarbon feedstock is fed to an oxygen or air-blown pressurized gasifier or reformer, where it is converted to syngas. The syngas passes through a shift reactor which increases the CO2 and H2 content at high temperature and pressure. This stream is then cooled before being wetted by a solvent capable of absorbing CO2. There are thus two flows: one rich in CO2 and one in pure hydrogen. The CO2 stream can be compressed and transported as well as the H2 stream. Figure 2.2 shows the scheme of the process described. You can have different process schemes:

• different feedstocks such as coal, petcoke, fuel oils, municipal solid waste and biomass can be used;

• natural gas and other volatile components can be used with a reformer.

The solvents used are many, the most used are Selexol and MDEA (methyl-diethanolamine).

In other cases, membrane and pressure swing absorption are used

Figure 2.1: Post-combustion flow scheme.

Figure 2.2: Pre-combustion flow scheme. [25]

Low carbon emission oil refinery through post-combustion technology and its heat integration. Politecnico di Milano

Politecnico di Milano