Environmental impacts of hydrotreating processes for the production of clean fuels based on life cycle assessment

Environmental impacts of hydrotreating processes for the production of clean fuels based on life cycle assessment

Fuel 164 (2016) 352–360 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Environmental impacts of hydr...

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Fuel 164 (2016) 352–360

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Environmental impacts of hydrotreating processes for the production of clean fuels based on life cycle assessment Le Wu a, Yongzhong Liu a,b,⇑ a b

Department of Chemical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi’an, Shaanxi 710049, PR China

a r t i c l e

i n f o

Article history: Received 27 April 2015 Received in revised form 29 July 2015 Accepted 6 October 2015 Available online 22 October 2015 Keywords: Hydrotreating process Desulfurization Life cycle analysis Environmental impact Simulation

a b s t r a c t Hydrotreating processes are important for the production of clean fuels and ultra-clean fuels. However, these processes consume a considerable amount of energy and trigger environmental problems. The production of ultra-clean fuels may lead to increased environmental loads. In this work, environmental impacts of hydrotreating processes for the production of clean fuels are quantified by Eco-indicator 99, which is based on the life cycle analysis. A diesel hydrotreating process is taken as an example to investigate the environmental impacts of the process with various sulfur contents in diesel and feed oil, in which simulations are carried out on the platform of ASPEN Plus. The results indicate that the optimal sulfur content range in diesel is 167.3–191.7 ppm, where the environmental impacts of the diesel hydrotreating process are minimized and the cost of utilities is relatively low. Moreover, the environmental impacts exhibit an approximately linear relationship with the sulfur content in the feed oil. The results also show the decreased sulfur content in the clean fuel does not reduce the environmental impacts of clean fuel production by using the hydrotreating process. Consequently, much more attention should be focused on the environmental impacts of the production process of clean fuel rather than the final product of clean fuel. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Ultra-low sulfur regulations have become stringently enforced worldwide. The sulfur content in diesel is less than 15 ppm in most countries [1], and the sulfur content in diesel for vehicles is expected less than 10 ppm in China [2]. The hydrotreating (HDT) process is an important technology for the production of clean fuels and ultra-clean fuels. This process consumes a considerable amount of steam, fuel gas, hydrogen, electricity and other utilities. In addition, to satisfy the stricter environmental regulations, refineries must perform deep desulfurization by increasing the temperature and pressure of the HDT reactors [1,3], thereby leading to a considerable increase in the demand for utilities and increasing the environmental loads resulting from the emissions of CO2 and SO2 produced in the HDT unit. Furthermore, this process increases the environmental loads because of the additional inferior crude oil processed. Therefore, the environmental impacts resulting from factors other than the clean fuels themselves should ⇑ Corresponding author at: Department of Chemical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China. Tel.: +86 29 82664752; fax: +86 29 83237910. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.fuel.2015.10.017 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

be considered when analyzing the production of clean fuels; the environmental impacts of clean fuels and the HDT process should be simultaneously taken into consideration. To determine the optimal sulfur content in clean fuels, the effects of the sulfur content in the feed and in the final clean fuel and of the HDT process on the environment should be analyzed. At present, the influences of emissions from the HDT process on the environment, such as CO2 emissions, are primarily considered. Babusiaux [4] calculated the CO2 emissions from the fuels, steams and electricity in a refinery by developing a linear programming (LP) model based on a life cycle analysis. In addition, the marginal CO2 emissions were obtained. Elkamel [5] proposed a mixedinteger nonlinear programming (MINLP) model of the production plan in refineries and obtained the minimum CO2 emissions after satisfying the product requirements. In addition to considering the CO2 emissions in a refinery, Dang [6] and Peters [7] discussed the superiority of biodiesel obtained via fast pyrolysis and hydroprocessing on the net non-renewable energy and greenhouse gas reduction compared with traditional diesel production using a life cycle analysis. Kochaphum [8] compared the global warming potential and abiotic resource depletion potential between straight-run diesel and cracked diesel. Miranda-Galindo [9] proposed a

L. Wu, Y. Liu / Fuel 164 (2016) 352–360

353

Nomenclature C cu DAMu EI Hhs Hls Hfg IMPj,u LCIi mCO2 M H2 md melec mfg mhs mH2

the cost of utilities (CNY a1) the price of the utilities (CNY kg1) the damage factor of the utilities or pollutants (Pt kg1 or Pt kW h1) the environmental impacts of the process and products (Pt a1) the enthalpy of the high pressure steam (kJ kg1) the enthalpy of the low pressure steam (kJ kg1) the calorific value of the fuel gas (kJ kg1) impact factor of utility or pollutant of each damage category (Pt kg1 or Pt kW h1) the impact factor of the utilities or pollutants of each impact category (Pt kg1 or Pt kW h1) the CO2 emission (kg h1) the molecular weight of hydrogen (kg kmol1) the flow rate of diesel (kg h1) the consumption of electricity (kW h) the consumption of fuel gas (kg h1) the consumption of high pressure gas (kg h1) the consumption of hydrogen (kg h1)

multi-objective optimization model for a hydrodesulfurization process for diesel using distillation with a side reactor to minimize the total annual cost (TAC), CO2 emissions and amount of sulfur compounds. The results indicated that the reduction in sulfur compounds increases the TAC and CO2 emissions. Furthermore, this process increased the environmental loads because of the inferior crude oil. Bredeson [10] analyzed the variations in CO2 emissions caused by crude oil with six different heaviness levels using the Shell Refinery Simulation Model. The results demonstrated that the CO2 emissions from the refinery increase as the heaviness of the crude oil increases. However, all of the aforementioned studies primarily considered the environmental impacts of the diesel HDT process; little attention has been paid to the comprehensive influence of the diesel HDT process and the product quality on environment. In other words, the previous works ignored the comprehensive influence of the diesel HDT process and the sulfur in product on the environment, particularly when the sulfur content in the feed and products change. Life cycle assessment (LCA) is widely used to assess the environmental impacts of goods and processes from ‘‘cradle to grave.” [11]. For the major methods of life cycle assessment, there are some frequently used methods, such as EDIP97, CML2001 and Eco-indicator 99. All of these methods can be used to quantitatively assess the environmental impacts of a product or a process by using LCA principles. EDIP97 and CML2001 are both midpoint approaches, whereas Eco-indicator 99 is an endpoint method [12], which is damage-oriented to quantitatively assess the environmental impacts of a process, including all of the materials and energy involved by using LCA principles [13]. It is widely used in the process assessment [14,15] and product analysis [16] in chemical processes. Subsequently, Eco-indicator 99 is also suitable to quantitatively assess the environmental impacts of the HDT process and to assess the comprehensive influence of the diesel HDT process and sulfur in product on the environment. In this work, we evaluate and analyze the environmental impacts of the HDT process and sulfur in diesel by using LCA principles. We conduct in-depth investigations on (1) the environmental impacts of the HDT process when the sulfur content in the feed changes and on (2) the environmental impacts of the HDT process when the sulfur content in the product changes. The environmental impacts of the HDT process are investigated with varying sulfur contents in the diesel and feed oil by simulations to obtain data for

mSO2 mu nH2 Wchc Wr Welec Wf wd,s wfg,c wfg,s

the SO2 emission (kg h1) the consumption or emission of utilities or pollutants (kg h1) the consumption of hydrogen (kmol h1) the power of cycle hydrogen compressor (kW) the power of the reboiler (kW) the power of equipment (kW) the power of the furnace (kW) the sulfur content in diesel (%) the carbon content in fuel gas (%) the sulfur content in fuel gas (%)

Greek

g

efficiency

Subscripts i impact categories j ecosystem qualities, human health and resources u utilities or pollutants

different degrees of desulfurization. The remainder of this paper is organized as follows. Section 2 describes the computation of the environmental impacts by using Eco-indicator 99. In Section 3, the simulation of a diesel HDT process via ASPEN Plus is presented. Section 4 discusses the environmental impacts of the diesel HDT process under varying sulfur contents in the diesel and feed and analyzes the cost of utilities. Finally, Section 5 presents the conclusions of this work.

2. Quantitative calculation of the environmental impacts of an HDT process 2.1. Typical HDT process Fig. 1 presents a typical HDT process. In this process, the feed is treated under high temperature and high pressure to remove sulfur, nitrogen, metals and other impurities. The refined products or clean fuels are obtained through this process by improving the molecular structure of the hydrocarbon. The input and output streams in this process are shown in Fig. 1. The feed and the utilities are in the input streams. The utilities include the high-pressure steam used to drive the cycle hydrogen compressor and the fuel gas combusted in the furnace and reboiler. Other utilities include hydrogen, electricity, desalted water, a poor methyldiethanolamine (MDEA) solution and stripping steam. The output streams include the products, low-pressure steam, a rich MDEA solution, sour water and the CO2 and SO2 from the furnace and reboiler. Generally, the flow rates of the streams in Fig. 1 are constant in the operating range of the HDT process. The solid-line box in the figure is the evaluation scope of the HDT process.

2.2. Quantitative evaluation of environmental impacts Eco-indicator 99 is used to quantify the environmental impacts of the HDT process and the products. The impacts of the products are primarily considered to be the impacts of SO2 from the complete combustion of the products because of the large variations in the sulfur content in the products and the small variation in other elements in the products. The environmental impacts of the sour water treatment unit and the MDEA recovery unit are

354

L. Wu, Y. Liu / Fuel 164 (2016) 352–360

Fig. 1. A typical HDT process.

Goal and scope definition

Environmental impacts of a diesel HDT process

Inventory analysis

The input and output of the process

Process impacts Product impacts

Table 1 The damage categories and the impact categories in Eco-indicator 99. Damage categories

Impact categories

Ecosystem quality

Acidification & eutrophication Ecotoxicity Land occupation

Human health

Carcinogens Climate change Ionizing radiation Ozone layer depletion Respiratory effects

Resources

Fossil fuels Mineral extraction

Utilities Sulfur in product Resources

Impact assessment

The environmental impacts of process

Human health Ecosystems conclusions

Interpretation

Assessment and analysis suggestions

Fig. 2. The calculation flowchart of Eco-indicator 99.

calculated based on the variation in the utilities consumed in the two units. On the basis of the evaluation scope, as shown in Fig. 1, and the Eco-indicator method [13], the environmental impacts of a HDT process and its products can be evaluated by four main steps as follows, as shown in Fig. 2. 1. Goal and scope definition This step defines the goal of the work, system boundaries, allocation methods and impact categories. In this study, we aim at reducing the environmental impacts of the HDT process and its products. These environmental impacts include two parts, the utilities consumed in the HDT process and the emissions by the products burning. According to Eco-indicator 99, ten impact categories are considered in this study, as listed in Table 1. 2. Inventory analysis In this phase, the relevant inputs and outputs of mass and energy associated with the HDT process are used to calculate the environmental impacts. The impacts contain the utilities consumed in the HDT process and the emissions by the products burning, SO2 and CO2, for example. The life cycle inventory (LCI) of the relevant utilities and pollutants can be adopted from the database of Eco-indicator 99, as listed in Table 2.

3. Impact assessment In this step, the life cycle inventory is translated into the corresponding environmental damage categories, which are human health, ecosystem quality and resources. The damage in each impact category is calculated from the life cycle inventory and the impact model. That is

IMPj;u ¼

X mu LCIi;u

8j

ð1Þ

i

where IMPj,u denotes the impact factor of the utilities or pollutants in each damage category, in Pt kg1 or Pt kW h1; and the subscript j represents the three damage categories, including ecosystem quality, human health and resources. The three damage categories contain ten impact categories, which are denoted by the subscript i. The specific definition can be obtained in the database of Eco-indicator 99. mu denotes the consumption or emissions of the utilities or pollutants, in kg h1 or kW h h1; the subscript u indicates the utilities or the pollutants; LCIi denotes the impact factor of utilities or pollutants of each impact category obtained from the database of the Eco-indicator 99, in Pt kg1 or Pt kW h1; The ten impact categories are aggregated into three damage categories, which are further translated into a single metric EI, the environmental impacts of the HDT process and the products.

DAMu ¼

X IMP j;u

ð2Þ

j

EI ¼

! X DAMu t u

ð3Þ

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L. Wu, Y. Liu / Fuel 164 (2016) 352–360 Table 2 The impact factors of the utilities or pollutants. H2 (Pt kg1) Acidification & eutrophication Ecotoxicity Land occupation Carcinogens Climate change Ionizing radiation Ozone layer depletion Respiratory effects Fossil fuels Mineral extraction a

Heata (Pt kJ1) 3

1.1111  10 2.4384  104 4.1414  105 9.4900  104 8.7019  103 2.0240  105 9.4230  108 1.1796  102 2.1437  101 7.7640  105

4

1.9332  10 1.1939  104 6.8154  105 6.8900  104 7.0870  104 2.1270  106 1.4460  108 2.4229  103 4.9960  104 3.1780  106

Electricity (Pt kW h1) 3

Fuel gas (Pt kg1) 3

2.8227  10 9.2960  104 1.1300  103 8.3630  103 7.5060  103 1.6482  106 5.7015  108 4.0248  102 3.8200  103 4.0958  105

2.3369  10 1.0089  103 2.8250  103 1.3602  103 1.8578  102 3.5458  105 1.2867  105 2.7264  102 1.7459  102 9.9051  105

CO2 (Pt kg1) 4

4.7560  10 1.9663  103 4.5080  104 6.2717  103 4.2075  103 1.0741  104 1.2980  106 6.8087  103 2.1285  102 1.0927  103

SO2 (Pt kg1) 5.6217  103 1.7910  103 4.7080  104 5.7086  103 2.2719  103 9.3018  105 9.5762  107 9.6813  102 1.3047  102 1.0510  103

The impact factors of high pressure steam and low pressure steam are calculated according to their enthalpies.

where DAMu represents the damage factor of the utilities or pollutants, in Pt kg1 or Pt kW h1; EI is the environmental impacts of the HDT process and the products, in Pt a1; t is the annual operating time, in h a1.

where mfg denotes the fuel gas consumption, in kg h1; Wf and Wr are the powers of the furnace and the reboiler, respectively, in kW; Hfg denotes the calorific value of the fuel gas, in kJ kg1; and gf and gr denote the heating efficiencies of the furnace and the reboiler, respectively.

4. Interpretation 3. Hydrogen consumption The results of the LCA are then analyzed in this step. And a set of conclusions and recommendations for the HDT process are proposed. 2.3. Calculations of the utilities consumed in the HDT process

Hydrogen is used to remove sulfur, nitrogen, metals and other impurities to upgrade the quality of the products. The hydrogen consumption is expressed as:

mH2 ¼ nH2 M H2

ð7Þ 1

The major utilities consumed in the HDT process are steam, fuel gas, hydrogen and electricity. The calculations of the utilities consumed are discussed in this section.

where mH2 denotes the hydrogen consumption, in kg h ; nH2 represents the flow rate of the hydrogen stream, in kmol h1; and MH2 is the molecular weight of hydrogen, in kg kmol1.

1. Steam consumption

4. Electricity consumption

High-pressure steam is used to drive the steam turbine to increase the pressure of cycle hydrogen with a cycle hydrogen compressor in the HDT process. The exhaust steam is discharged into the low-pressure steam header. The high-pressure steam consumption is expressed as [17]:

Electricity drives the pumps, air coolers and the make-up hydrogen compressor. The electricity consumption is expressed as:

mhs ¼ W chc =½ðHhs  Hls Þgt 

ð4Þ

where mhs denotes the high-pressure steam consumption, in kg h1; Wchc indicates the power of the cycle hydrogen compressor, in kW; Hhs and Hls are the enthalpies of the high-pressure steam and the low-pressure steam, respectively, in kJ kg1; and gt is the overall efficiency of the steam turbine. The low-pressure steam consumption is equal to the highpressure steam consumption after working if the loss is ignored. The low-pressure steam discharged to the low-pressure steam header is shown as:

mls ¼ mhs

ð5Þ

where mls denotes the low-pressure steam discharged to the lowpressure steam header, in kg h1. 2. Fuel gas consumption The fuel gas is combusted in the furnace and the reboiler of the fractionator in the HDT process. The feed is heated to the target temperature in the furnace before being delivered to the hydrogenator. The reboiler heats the liquid material to provide a certain amount of rising vapor to guarantee stable operation of the fractionator. The fuel gas consumption contains two parts:

mfg ¼ W f =ðHfg gf Þ þ W r =ðHfg gr Þ

ð6Þ

melec ¼

X

ð8Þ

W elec t

where melec indicates the total electricity consumption, in kW h, and Welec denotes the electricity consumed by each piece of equipment, in kW. 2.4. Calculations of CO2 and SO2 emissions in the HDT process In the HDT process, the burning of fuel gas discharges a substantial amount of CO2 and a minimal amount of SO2. For the fuel product, the combustion of the products discharges a certain amount of CO2 and SO2 which can cause damage to the environment. The CO2 emitted from the combusted fuel gas is only considered due to the minimal variation with respect to the carbon content in the products. The CO2 emissions are expressed as:

mCO2 ¼ 44=12  mfg wfg;C

ð9Þ 1

where mCO2 is the CO2 emitted, in kg h ; 44 and 12 denote the molecular weights of CO2 and carbon, respectively, in kg kmol1; and wfg,C indicates the carbon content in the fuel gas, expressed as %. SO2 is primarily emitted from the combustion of fuel gas and products. The amount of emission can be expressed as:

mSO2 ¼ 64=32  ðmfg wfg;S þ md wd;S Þ

ð10Þ 1

where mSO2 is the SO2 emitted, in kg h ; 64 and 32 denote the molecular weights of SO2 and sulfur, respectively, in kg kmol1; wfg,S and wd,S are the sulfur contents of the fuel gas and the products, respectively, expressed as %; and md is the flow rate of products, in kg h1.

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L. Wu, Y. Liu / Fuel 164 (2016) 352–360

3. Simulations of a diesel HDT process Taking a diesel HDT process with the annual processing capacity of 3.2 million tons as an example, the comprehensive impacts of the diesel HDT process and diesel on the environment are clarified. The diesel HDT process is simulated by using ASPEN Plus (Version 7.3) to obtain the fundamental data. These data are used to calculate the environmental impacts, including the utilities consumed in the diesel HDT process. The simulation conditions are in strict accordance with the actual conditions. According to the user guide [18] of ASPEN Plus, Grayson is selected as the property method and ENRTL-HG is selected as the property method when electrolytes are present in the system; these selections are based on the properties of the diesel HDT process. The reactions in a diesel HDT process include hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodearomatization (HDA), hydrodeoxygenation, hydrodemetallization and hydrocracking. HDS, HDN and HDA are primarily considered in the simulations. The lumped reaction model proposed by Zavala [19] is selected to describe the kinetics of HDS. The specific data of the lumped reaction model were reported by Froment [20] and Houalla [21]. The kinetics of HDN is modeled in the work of Liang [22] and Bej [23]. The kinetics of HDA is modeled in the work reported by Cheng [24]. The rate constants of each reaction are modified based on industrial data. As shown in Fig. 3, the feed and hydrogen are mixed, and then react in the trickle bed reactor (TBR) (Reactor 1) after the temperature and pressure are increased. Cold hydrogen is used to decrease the temperature of the stream when entering the second reactor bed (Reactor 2). The stream from the reactor is cooled by a heat exchanger and then enters the hot high-pressure flash separator (Separator 1), hot low-pressure flash separator (Separator 2), cold high-pressure flash separator (Separator 3) and cold lowpressure flash separator (Separator 4). After separation, the hydrogen-rich gas stream contacts with MDEA to remove

hydrogen sulfide and then enters the cycle hydrogen compressor for recycling. The liquid from the separators enters the stripper to remove the hydrogen sulfide and other impurities dissolved in the liquid. The liquid from the bottom of the stripper then enters the fractionator to extract the diesel from the bottom and naphtha from the top. The primary operating parameters of main equipment are shown in Fig. 1. We compare the simulation results with the actual data. Table 3 shows the main operating parameters and the utilities consumed. Table 4 presents a comparison of the properties of the streams. As shown in these tables, the results of the simulations conducted in this work are in close agreement with the actual industrial results. Hence, the simulations provide a good foundation for the follow-up work. 4. Environmental impacts and economic analysis of the diesel HDT process In this section, we will discuss the environmental impacts of the HDT process under two typical scenarios. One is that the sulfur content in the feed changes while the sulfur content in the product remain constant, and another one is that the sulfur content in the product is variable while the sulfur content in the feed remain constant. In addition, the economic analysis is carried out thereafter. 4.1. Scenario 1: sulfur content changes in feed According to actual industry data, the diesel HDT process is simulated when the sulfur content in the feed is 1.3%, 1.4%, 1.5%, 1.7% and 1.8% based on the simulation of 1.57% sulfur in the feed of the diesel HDT process. The flow rate of the feed is invariable, and the reaction pressure is maintained at 7.7 MPa during the simulation. The utilities consumed by the process are shown in Table 5. The operating range of the process is 60–120%. The flow rate of the

Make-up H2 comp

Cycle H2 comp

H2

Reactor 1

MDEA

Absorber

Reactor 2

Separator 3 Separator 4 Separator 1 Separator 2

Naphtha Fractionator

Stripper

Diesel

Feed Fig. 3. The simulation of a diesel HDT process.

Table 3 The comparison of the main operating parameters and the utilities consumed.

Actual Simulation

The inlet/outlet temperature of Reactor 1 (°C)

The inlet/outlet temperature of Reactor 2 (°C)

Pressure (MPa)

H2/oil ratio

Make-up H2 (N m3 h1)

The top, feed and bottom temperatures of fractionator (°C)

Cycle H2 comp (kW)

Electricity (kW)

Furnace (kW) Reactor

Fractionator

318.0/343.0 318.0/343.5

335.0/355.0 336.0/355.7

7.7 7.7

300 283

33,443 33,779

161/251/302 152/251/296

1960 1829

6381.5 6266.7

6485 6315

24,462 24,004

357

L. Wu, Y. Liu / Fuel 164 (2016) 352–360 Table 4 The comparison of the properties.

Feed 1

Feed 2

Feed 3

Diesel

Naphtha

a

Actual Simulation Deviation (%) Actual Simulation Deviation (%) Actual Simulation Deviation (%) Actual Simulation Deviation (%) Actual Simulation Deviation (%)

Flow rate (t h1)

Densitya (kg m3)

283.2 283.2 0 40.9 40.9 0 56.8 56.8 0 333.7 334.0 0.1 41.2 38.1 7.2

789.1 800.3 1.4 683.5 686.7 0.5 790.2 826.9 4.6 618.5 617.3 0.2 717.2 706.4 1.5

Distillation curve (°C) IBP

10

30

50

70

90

FBP

253 255 0.8 48 46 4.2 188 184 2.1 188 187 0.5 45 45 0

256 261 2.0 74 73 1.4 214 213 0.5 231 254 10 74 87 17.6

269 279 3.7 96 98 2.1 242 247 2.1 252 264 4.8 95 99 4.2

285 292 2.5 117 118 0.9 272 279 2.5 274 279 1.8 116 119 2.6

307 312 1.6 138 139 0.7 302 314 4.0 297 300 1.0 137 137 0

327 326 0.3 159 165 3.8 331 330 0.3 330 324 1.8 159 160 0.6

341 344 0.9 181 183 1.1 350 358 2.3 355 347 2.3 180 183 1.7

S (ppm)

N (ppm)

14,900 14,901 0 7000 7009 0.1 26,000 26,018 0 40 38.2 4.5 <10 0 /

42 42 0 110 110 0 750 750 0 15 14.2 5.3 <5 0 /

The density under the operating conditions.

Table 5 The utilities consumed in the process.

1.3 1.4 1.5 1.57 1.7 1.8

Inlet temp (°C)

320.3 319.0 318.5 318.0 316.6 316.0

Make-up H2 (N m3 h1)

30,419 31,651 32,906 33,779 35,392 36,624

Cycle H2 comp (kW)

1829 1829 1829 1829 1829 1829

desalted water, poor MDEA solution and stripping steam remain unchanged because the variation in the sulfur content in the feed is within the range of operating flexibility. Additionally, the variations in the utilities consumed in the sour water treatment unit and the MDEA recovery unit are ignored. The enthalpies [17] of high-pressure steam, low-pressure steam and fuel gas of the diesel HDT process are 3189.1, 2777.1 and 4.658  107 kJ kg1, respectively. According to the actual data, the overall efficiency, gt, of the steam turbine driving the cycle hydrogen compressor is 64.6%, and the heating efficiency of the furnace and the reboiler is 93.8%. The carbon content in the fuel gas is 66.3%, and the sulfur content is 18.8 ppm. The annual operating time is 8400 h. The utilities consumed and the CO2 and SO2 emitted can be calculated according to the data presented in Tables 3 and 5. The environmental impacts caused by the utilities and by the pollutants can be calculated according to the database of Eco-indicator 99, as shown in Table 2 [25]. Fig. 4 shows the effect of changing the sulfur content in the feed on the environmental impacts. The impact on ecosystem quality remains unchanged with variations in the sulfur content in the feed, whereas the impacts on human health and resources present an approximately linear relationship with the sulfur content in the feed. As shown in Fig. 4, the total environmental impacts increase from 16.45 MPt a1 to 17.11 MPt a1 when the sulfur content in the feed increases from 1.3% to 1.8%. When the sulfur content in the feed is 1.5%, the impact on resources presents the largest value of 62.3% because the production of utilities consumes a considerable amount of energy. The proportion of the human health impact is 34.1%, and the smallest proportion is the impact on ecosystem quality, i.e. 3.6%. Fig. 5 presents the proportions of the utilities and the pollutants when the sulfur content in the feed changes. With increasing sulfur

Elec (kW)

6002.1 6098.7 6196.8 6266.7 6392.5 6489.2

Furnace (kW) Reactor

Fractionator

7236 6720 6517 6315 5760 5518

24,715 24,472 24,194 24,004 23,536 23,305

Diesel (t h1)

S in diesel (ppm)

334.0 334.0 334.0 334.0 334.0 334.0

38.6 38.5 38 38.2 38.3 38.9

18 Resources

Human health

17

EI / MPt⋅a-1

S in feed (wt%)

Ecosystems

1.57 %(wt)

16 6

3

0 1.3

1.4

1.5

1.6

1.7

1.8

Sulfur in feed / % (wt) Fig. 4. The effect of the changing sulfur content in the feed on the environmental impacts.

contents in the feed, the impacts of electricity and hydrogen increase, the impacts of fuel gas and CO2 decrease, and the impacts of steam and SO2 remain unchanged. The increasing sulfur content in the feed consumes more hydrogen. Additional hydrogen increases the amount of electricity consumed to drive the make-up hydrogen compressor. The reaction will release more heat when the sulfur in the feed increases because the process is an exothermic reaction. This release of heat will lead to a decrease in the inlet temperature and then decrease the consumption of fuel gas and the amount of CO2 emitted. The cycle hydrogen and the sulfur in the diesel remain unchanged, producing stable steam consumption and SO2 emissions. When the

L. Wu, Y. Liu / Fuel 164 (2016) 352–360

8

H2 SO2

Steam CO2

Elec

Fuel gas

EI / MPt⋅a-1

6

4

2

0 1.3%

1.5%

1.7%

Sulfur in feed / % (wt) Fig. 5. The effect of the sulfur content in the feed on the impacts of the utilities and pollutants.

sulfur content in the feed is 1.5%, hydrogen accounts for the largest proportion, i.e. 35.7%, owing to the considerable amount of hydrogen consumed in the HDT process. This proportion is followed by the electricity, fuel gas and steam. The impacts of the pollutants are primarily caused by the CO2 emissions, whereas SO2 accounts for the smallest proportion of impacts, i.e. 0.2%. Therefore, the impacts caused by the diesel HDT process are the main impacts on the environment. The impacts of the HDT process can be alleviated by reducing the consumptions of fuel gas, hydrogen and electricity in this system. For examples, the consumptions of the utilities and hydrogen can be optimized by Pinch analysis technologies of heat exchanger network and hydrogen distribution network for the HDT process. To further reduce the hydrogen consumption for the production of clean diesel, some nonhydrodesulfurization technologies could be used, such as oxygen oxidation desulfurization, ultrasonic oxidation desulfurization, photocatalytic oxidation desulfurization and biological desulfurization. These technologies can be potentially used in industries as viable technologies to replace the classical hydrodesulfurization. 4.2. Scenario 2: sulfur content changes in product

The environmental impacts of the different product qualities are shown in Fig. 6. The impact on the ecosystem quality remains unchanged with variations in the sulfur content in the diesel, whereas the impacts on human health and resources decrease when the sulfur content in diesel increases. The total environmental impacts first decrease, then reach a minimum and finally are approximately constant with increases in the sulfur content in diesel. As shown in Fig. 6, the total environmental impacts are approximately 15.83 MPt a1 when the sulfur content in the product diesel exceeds 170 ppm. The proportion of the impact on resources is 62.7%, that on human health is 33.8%, and that on ecosystem quality is 3.5%. When ultra-clean fuel, in which the sulfur content is 7.4 ppm, is produced, the total environmental impacts reach 18.14 MPt a1. When the sulfur content in diesel ranges from 167.3 to 191.7 ppm, the total environmental impacts are relatively lower at 15.86 MPt a1. This range is the optimal sulfur content for the product diesel. It implies the smaller environmental impacts for both the product diesel and the production process for the production of the clean fuels. Therefore, when the environmental impacts of the HDT process and product quality are both considered, less sulfur in the diesel does not necessarily result in fewer impacts on the environment. In other words, when the HDT process is used to produce clean fuels by increasing the temperature and pressure to achieve a more thorough desulfurization, the environmental impacts sharply increase because of the increase in the utilities consumption. It reveals that simply reducing the impacts of the product (sulfur content in diesel) may not be appropriate if the impacts of the production process are ignored. Fig. 7 presents the proportions of the utilities and the pollutants when the sulfur content in diesel changes. The impacts of the 20

Resources

Human health

Ecosystems

7.4 38.2

75.2 104.1 132.6 167.3 191.7

228.3

266.3

318.6

15

EI / MPt⋅a-1

358

10

5

4.2.1. Environmental impacts of the HDT process The environmental impacts of the production of different product qualities are studied in this section to discuss the optimal sulfur content in clean fuels by considering the impacts of both the HDT process and the product qualities. The production processes for products with different qualities are simulated. The data are summarized in Table 6.

0 0

50

100

150

200

250

300

350

Sulfur in diesel / ppm Fig. 6. The environmental impacts of the different product qualities.

Table 6 The utilities of the different product qualities. S in diesel (ppm)

Inlet temperature (°C)

Pressure (MPa)

Make-up H2 (N m3 h1)

Cycle H2 comp (kW)

Electricity (kW)

7.4 38.2 75.2 104.1 132.6 167.3 191.7 228.3 266.3 318.6

325.0 318.0 314.0 312.5 311.0 310.0 309.5 309.0 308.5 308.0

8.1 7.7 7.6 7.5 7.4 7.3 7.3 7.3 7.3 7.3

33,802 33,779 33,690 33,667 33,600 33,555 33,510 33,443 33,376 33,286

2679 1829 1627 1394 1241 1070 1070 1070 1070 1070

6481.9 6266.7 6204.2 6148.5 6087.7 6030.1 6026.6 6021.3 6016.3 6009.3

Diesel (t h1)

Furnace (kW) Reactor

Fractionator

9160 6315 4758 4193 3619 3249 3062 2875 2689 2502

25,661 24,004 23,982 24,022 24,061 24,064 24,046 24,083 24,094 24,107

334.0 334.0 334.0 334.0 334.0 334.0 334.0 334.0 334.0 334.0

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L. Wu, Y. Liu / Fuel 164 (2016) 352–360

H2 SO2

Steam CO2

Elec

200

Fuel gas

Fuel gas

Opeatational cost / Million CNY⋅ a-1

8

EI / MPt⋅a-1

6

4

2

0 7.4 ppm

38.2 ppm

180

38.2

75.2 104.1 132.6 167.3 191.7

228.3

266.3

318.6

120

0 50

100

150

200

250

300

Sulfur in diesel / ppm

Fig. 7. The effect of the sulfur content in diesel on the impacts of the utilities and pollutants.

utilities and CO2 decrease when the sulfur content in the diesel increases from 7.4 to 167.3 ppm because the consumption of utilities and the CO2 emissions are reduced. When the sulfur content in diesel is 38.2 ppm, hydrogen accounts for the largest proportion, i.e. 35.7%, because of the considerable amount of hydrogen consumed in the HDT process, corresponding to the environmental impact of 6.01 MPt a1. The impact of hydrogen is followed by the impacts of the fuel gas, electricity, CO2 and steam. The SO2 accounts for the smallest proportion, i.e. 0.2%; the environmental impact is 0.03 MPt a1 for a SO2 emission of 0.22 kt a1. The impact caused by SO2 increases to 0.12 MPt a1 when the sulfur content is 167.3 ppm, which is located in the optimal range. However, the environmental impact of SO2 is relatively lower than that of the other factors. Therefore, the impacts of the HDT process can be effectively reduced by decreasing the consumption of hydrogen, fuel gas and electricity. 4.2.2. Economic analysis of the HDT process The cost of utilities of the diesel HDT process is analyzed to further clarify the relationship between the degree of desulfurization and economics. The cost of utilities and the environmental impacts are compared when the sulfur content in the diesel changes. The prices of the hydrogen, electricity, high-pressure steam, lowpressure steam and fuel gas are 5.406 CNY kg1, 0.52 CNY kW h1, 0.12 CNY kg1, 0.09 CNY kg1 and 0.243 CNY kg1, respectively. The prices of the utilities are adopted from a refinery of China in 2014. The cost of the utilities in the diesel HDT process include hydrogen, electricity, steam and fuel gas, which can be expressed as



H2

140

Sulfur in diesel / ppm

! X cu mu  cls mls t

Steam

160

0

167.3 ppm

Elec

7.4

ð11Þ

u

where C represents the cost of utilities, in CNY a1; cu denotes the price of each utility, in CNY kg1 or CNY kW h1; and ls is the abbreviation for low pressure steam. It should be noting that the lowpressure exhaust steam from the steam turbine should be removed when the cost of utilities is calculated. Fig. 8 presents the relationship between the cost of utilities and the sulfur content in diesel. The effect of the sulfur content in diesel on the cost of utilities is similar to the effect on the environmental impacts. The cost of utilities first sharply decreases and then gently decreases when the sulfur content in the diesel increases from 7.4 to 318.6 ppm. As shown in Fig. 8, two points exhibit large variations in gradient: 38.2 ppm and 167.3 ppm.

Fig. 8. The relationship between the cost of utilities and the sulfur content in diesel.

When the sulfur content in the diesel increases from 7.4 to 38.2 ppm, the cost sharply decreases. When the sulfur content in the diesel exceeds 167.3 ppm, the cost gradually decreases. Therefore, producing a diesel with a sulfur content of 7.4 ppm is uneconomical in this diesel HDT process. However, producing a diesel with a sulfur content of 167.3 ppm is reasonably economical. Fig. 8 presents the proportions of the utilities in the cost of utilities when the sulfur content in the diesel changes. The cost of utilities decreases as the sulfur content in the diesel increases. The hydrogen consumption is directly related to the sulfur content in the diesel, and it decreases as the sulfur content in the diesel increases from 7.4 to 167.3 ppm. The steam consumption decreases sharply because of the decrease in the reaction pressure. The reaction pressure also affects the consumption of electricity used to drive the make-up hydrogen compressor. The reaction temperature affects the fuel gas consumption. When the sulfur content in the diesel is 38.2 ppm, hydrogen accounts for the largest proportion, i.e. 78.0%, of the cost because of the considerable consumption in the HDT process, corresponding to a cost of 1.37  108 CNY a1. The cost of hydrogen is followed by the cost of electricity, at 2.74  107 CNY a1, in which the proportion is 15.6%. The proportions for the steam and fuel gas are small, at 3.5% and 2.9%, respectively. The costs for the steam and fuel are 6.23  106 CNY a1 and 5.10  106 CNY a1, respectively. 5. Conclusions The HDT process is an important technology for producing clean fuels and ultra-clean fuels. However, the production process of ultra-clean fuels may increase the environmental loads, whereas the product is clean. In this paper, Eco-indicator 99 is used to quantify the environmental impacts of HDT. The environmental impacts are studied under the various sulfur contents in diesel and feed via simulations. Taking a diesel HDT process with the annual processing capacity of 3.2 million tons as an example, the results of the simulations conducted using ASPEN Plus closely correspond with actual industry data. The diesel HDT process is simulated when the sulfur content in the feed and diesel changes based on actual industry data. The utilities consumed and the pollutants emitted are calculated. Eco-indicator 99 is adopted to quantitatively assess the environmental impacts of the HDT process and to assess the comprehensive influence of the diesel HDT process and sulfur contents in the diesel and feed on the environment. The environmental impacts increase when the sulfur content in the feed increases

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from 1.3% to 1.8%. When the sulfur content in the feed increases by 0.1%, the environmental impacts increase by 0.8%. The environmental impacts of the diesel HDT process are minimized and the cost of utilities is relatively low when the sulfur content in the diesel ranges from 167.3 ppm to 191.7 ppm. When 10 ppm sulfur in the product diesel is achieved by increasing the reaction temperature and pressure of the diesel HDT process, the environmental impacts and cost of utilities are maximum. When the sulfur content in the diesel is 38.2 ppm, the impacts of the hydrogen consumption account for the largest proportion, and the least proportion is accounted for by the SO2 emissions from the diesel and fuel gas combustion. The hydrogen consumption also contributes the largest proportion of the cost of utilities, i.e. 78%, whereas the fuel gas accounts for only 2.9%. On the basis of the abovementioned analysis, more attention should be paid to the environmental impacts of clean fuel production processes. Environmental impacts other than those of the end products should be considered. The environmental impacts present an approximately linear relationship with the sulfur content in the feed. An optimal sulfur content range is observed for the diesel in the HDT unit. In this range, the environmental impacts are minimized and the cost of utilities is relatively low. Consequently, from the perspective of the entire life cycle of the product, the impacts of the production process for clean or ultra-clean fuel should be considered when determining the product quality of a HDT process. In addition, the HDT process available could be optimized to reduce the consumption of the utilities if the environmental impacts of the production process for ultra-clean fuel are expected to be reduced further. Acknowledgments The authors gratefully acknowledge funding by the projects (Nos. 21376188 and 21176198) sponsored by the Natural Science Foundation of China (NSFC) and the Industrial Science & Technology Planning Project of Shaanxi Province (No. 2015GY095). References [1] Stanislaus A, Marafi A, Rana MS. Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production. Catal Today 2010;153:1–68. [2] State Standard of the People’s Republic of China. Automobile diesel fuels (GB 19147-2013). Beijing: Standardization Administration of the People’s Republic of China; 2013. [3] Murali C, Voolapalli R, Ravichander N, Gokak D, Choudary N. Trickle bed reactor model to simulate the performance of commercial diesel hydrotreating unit. Fuel 2007;86:1176–84. [4] Babusiaux D, Pierru A. Modelling and allocation of CO2 emissions in a multiproduct industry: the case of oil refining. Appl Energy 2007;84:828–41.

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