Multi-objective Operational Optimization of a Hydrotreating Process

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Multi-objective Operational Optimization of a Hydrotreating Process Based on Hydrogenation Reaction Kinetics Le Wu, Yuqi Wang, Lan Zheng, Xiaolong Han, and Furong Hong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03379 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Industrial & Engineering Chemistry Research

Multi-objective Operational Optimization of a Hydrotreating Process Based on Hydrogenation Reaction Kinetics

Le Wua, b, *, Yuqi Wanga, b, Lan Zhenga, b, Xiaolong Hana, b, Furong Hongc

a

Department of Chemical Engineering, Northwest University, Xi'an 710069, China

b

Shaanxi Provincial Institute of Energy Resource & Chemical Engineering, Xi'an 710069, China

c

CNOOC Ningbo Daxie Petrochemical Ltd, CNOOC, Ningbo 315812, China

*Corresponding Author Tel:

+86-29-88302632

Fax:

+86-29-88302223

E-mail: [email protected]

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Utility consumptions in hydrotreating (HDT) units are rapidly increasing because more heavy and sour crude oil are being processed to satisfy the increasing market demand of clean fuels, which then cause the directly and indirectly emissions of CO2 and SO2. The environmental impacts of utilities should be considered when optimizing an HDT unit. A multi-objective optimization is proposed to reduce the operating cost and environmental impacts of utilities and products based on hydrogenation reaction kinetics. A cracked diesel HDT unit is adopted to illustrate the proposed model which is solved by ɛ-constrains method to obtain the Pareto fronts and three scenarios: minimum operating cost, minimum environmental impacts and the best compromise. For all scenarios, increasing temperature and decreasing pressure can reduce both the operating cost and environmental impacts. The decreasing of sulfur content in product is at the expense of consuming more utilities. It is not always favorable when the sulfur content of the product is decreasing. Thus there is an optimum desulfurization degree in the HDT unit when reducing the environmental impacts. Therefore, the environmental impacts of an HDT process, especially for the impacts of the production process, should not be ignored when optimizing an HDT process. The most efficient way to reduce both the operating cost and environmental impacts is to reduce the hydrogen consumption. Keywords: Hydrotreating process; Operational optimization; Life cycle assessment; Hydrogenation reaction kinetics; Multi-objective optimization


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1

Introduction The energy consumption and hydrogen consumption are increasing because more

heavy and sour crude oil are being processed to make clean fuels and ultra-clean fuels and satisfy the increasing market demands1. The CO2 and SO2 are directly and indirectly emitted due to the high energy and hydrogen consumptions in refineries. The environmental problems trigged by these pollutants should not be ignored due to the rapidly growth of utility consumptions 2, 3. Therefore, under the premise of meeting more stringent environmental regulations, it is important to reduce the energy and hydrogen consumptions and their environmental impacts in refineries. Considering the large utility consumptions of a hydrotreating (HDT) unit, Ahmad et al.4 built a nonlinear programming (NLP) to simultaneously optimize the diesel reactor, the distillation column and the heat integration system and which is solved by a simulated annealing optimization algorithm. The model can be used for an HDT process at conceptual design stage. Muñoz et al.5 optimized the heat exchanger network by modeling the whole operating scheme of a heavy oil HDT unit. Zhang et al.6 proposed a multi-period NLP to optimize a heat exchanger network integrating the material and energy balances and the price change of products in a diesel HDT unit. The utility consumptions in an HDT unit can be decreased by using the integration method. However, the hydrogenation reaction kinetics are ignored when optimizing an HDT unit in the above-mentioned studies. To integrate the kinetics and optimize the operating conditions, Al-Adwani et al.7



proposed a long-term optimization model for a residue oil HDT process integrating the hydrodesulfurization (HDS) kinetics and the catalyst deactivation. The relations among the catalyst cost, operating cost and desulfurization benefit were also studied. Bhran et al.8 adopted the quadratic-polynomial equations to represent the hydrogenation reaction kinetics which were then used to optimize the operating conditions of an HDT process and achieve maximum conversions of impurities in the feed oil. The optimization of a shale oil HDT process was conducted based on HDS, hydrodenitrogenation (HDN) and saturation of aromatics (HDA) kinetics for maximizing the transportation fuel yields with ultra-low sulfur and nitrogen.9 Peng et al.10 investigated the effects of different tail oil recycle schemes on the optimization of a vacuum gas oil hydrocracking process. Sbaaei and Ahmed11 proposed an HDT model in HYSYS to study the effects of different operating variables on process performance. Furthermore, the operating conditions of the HDT process were optimized to save the fuel and energy consumption and increase the process productivity. Wu et al.12 integrated a fluid catalytic cracking (FCC) unit to an HDT system consisting of a vacuum gas oil HDT unit, a cracked diesel HDT unit and a cracked gasoline HDT unit to optimize the operating conditions of these HDT units and discussed the effect of the FCC unit on the utility consumptions. However, these studies only considered to reduce the operating cost and obtain more ultra-low sulfur fuels by optimizing the operating conditions while the environmental impacts of an HDT unit are ignored. Then the obtained optimal operating conditions may


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not be accurate in a sustainable view. Furthermore, more attention are paid on the quality of fuels while less attention are paid on the environmental issues triggered by the emissions of CO2 and SO2 from the production processes of the ultra-low sulfur fuels. Thus, not only the environmental impacts of the fuels but also the impacts of the production process should be considered when reducing the utility consumptions and optimizing the operating conditions of an HDT process. A multi-objective operational optimization model is proposed to simultaneously minimize the operating cost and environmental impacts of an HDT process based on the hydrogenation reaction kinetics, in which the Eco-indicator 99 is used to quantify the environmental impacts according to the life cycle assessment. The effects of the sulfur content in feed oil and product on the operating cost and environmental impacts are also studied.

2

Multi-objective optimization model CO2，SO2

CO2, SO2, NOX

Steam

Recycling H2 Combustion

Electricity

Compressor

Hydrogen Compressor

Feed oil Electricity

Product Furnace

Pump

HDT reactor

Fuel gas

Fig 1 Utilities consumed and pollutants emitted in an HDT process



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According to the flowsheet of an HDT process in Fig 1, the HDT reactor provides the reaction zone for the feed oil and hydrogen under high temperature and pressure. The steam, electricity and fuel gas are consumed to increase the operating pressure and temperature and the hydrogen is consumed to remove the impurities like sulfur, nitrogen and aromatics to improve the product quality. A multi-objective optimization model is proposed to minimize the operating cost and environmental impacts of an HDT process by optimizing the operating conditions based on hydrogenation reaction kinetics. 2.1 2.1.1

Objective functions Economic objective According to Fig 1, the utilities consumed in an HDT process are mainly electricity,

steam, fuel gas and hydrogen. The economic objective in the proposed multi-objective optimization model is the operating cost consisting of the utility costs, labor cost and maintenance cost. The calculation of labor cost and maintenance cost are presented in Supporting Information.

min OC   mi pi AOT  CL  CM i

(1)

where OC denotes the operating cost, in US$y-1. m is the utility consumptions, in kW, MJh-1 and th-1; i represents the set of utilities; p denotes utility prices, in US$kWh-1, US$MJ-1 and US$t-1; AOT is the abbreviation of annual operating time, in hy-1. CL and CM are labor cost and maintenance cost, respectively, in US$y-1.


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2.1.2

Environmental objective During the productions of utilities, large amount of fossil fuels are consumed and then

emit a certain amount of CO2, which can cause the environmental problems like depleting of resources and greenhouse effect. Thus, the environmental impacts of utilities should be considered. Furthermore, the combustions of the fuel gas and HDT products emit CO2, SO2 and NOX whose environmental impacts also cannot be ignored. The environmental objective of this model is to minimize the environmental impacts of the utilities and pollutants, which is shown as followed:

min EI

(2)

where EI is the environmental impacts, in pty-1. 2.2

Environmental impacts of an HDT process According the methodology of life cycle assessment and Eco-indicator 9913, the

calculation of the environmental impacts can be divided to 4 steps which are shown in Fig 2.



Goal and scope definition

Environmental impacts of an HDT process

The input and output Inventory analysis of the process Impact assessment

Interpretation

Production process Product combustion Utilities & Pollutants Sulfur & Nitrogen in product

Total environmental impacts

Utility impacts

Assessment and analysis

Conclusions

Pollutant impacts

Suggestions

Fig 2 The environmental impacts of an HDT process

1) Goal and scope definition In this work, we are aiming to reduce the environmental impacts of the HDT process shown in Fig 1, the impacts are consisted of the production process impacts and the product combustion impacts. 2) Inventory analysis The input and output of the HDT process are defined in this step. The input streams contain the feed oil and the utilities consumed in the process. The output steams are mainly the pollutants from the fuel gas combustion and the product. The environmental impacts of the production process are utilities and pollutants. The impacts of the product are the impacts of CO2, SO2 and NOX from the product combustion. 3) Impact assessment The total environmental impacts of the HDT process is calculated in this phase.


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According the previous step, the total impacts are consisted of the utility impacts and pollutant impacts. 4) Interpretation A set of conclusions and suggestions can be obtained in this step according the results from step 3. According to Fig 2 and Eco-indicator 9913, the environmental impacts of an HDT process can be calculated by eq(3).   EI    mi DAM i   m j DAM j  AOT j  i 

(3)

where DAM denotes the damage factors of utilities and pollutants, in ptkWh-1, ptMJ-1 and ptt-1; j is the set of pollutants, CO2, SO2 and NOX. 2.3 2.3.1

Utility consumptions and pollutant emissions Utility consumptions (1) Electricity There are usually two drivers for pumps and compressors in a refinery, electric motor

and steam turbine, which consume electricity and high-pressure steam, respectively. The electricity consumption can be calculated by eq(4) if an electric motor is used as the driver.

melec  Wpump  Wcomp Wpump 

vfeed  Ppump,out  Ppump,in 

pump


(4) (5)


Wcomp

 1   C pTcomp,s  Pcomp,d        1 mcomp  comp  Pcomp,s   

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(6)

where the subscripts elec, pump, comp, feed, out, in, d and s denote electricity, pump, compressor, feed oil, outlet stream, inlet stream, discharge stream and suction stream of a compressor, respectively. W is the power of a pump or a compressor, in kW; v represents the volumetric flowrate of the feed oil, in m3h-1; P is pressure, in MPa; η denotes the total efficiency, in %; Cp is the constant-pressure specific heat, in Jmol-1K-1; Tcomp,s denotes the temperature of the suction stream in a compressor, in K; γ represents the specific heat ratio;

mcomp is the flowrate of the compressed gas, in mols-1. (2) High-pressure steam The high-pressure (HP) steam would be consumed if pumps and compressor are driven by steam turbines. The HP steam consumption is presented as below: mHS 

Wturb  H HS  H LS turb

(7)

where the subscripts HS, turb and LS are HP steam, steam turbine and low-pressure steam, respectively. H is the enthalpy, in MJt-1. (3) Low-pressure steam The low-pressure (LP) steam is discharged from a steam turbine after working and then flowed into a steam header for other uses. Thus, the LP steam cost should be subtracted from the total operating cost. The consumption of LP steam can be calculated by eq(8) if


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the leakage is ignored for the simplification of the multi-objective optimization model.

mLS =mHS

(8)

(4) Fuel gas The fuel gas is consumed to heat up the feed oil to the operating temperature. According to Wu and Liu’ study3, a linear function can be used to describe the relation between the heat load of a furnace and its outlet temperature.

Wfur  a Tout  b

(9)

mFG  Wfur H FG

(10)

where the subscripts fur and FG are furnace and fuel gas, respectively. (5) Hydrogen In an HDT unit, hydrogen is consumed to remove the sulfur, nitrogen and aromatics. Also, hydrogen can remove other impurities like oxygen, metals and olefins. In addition, the feed oil can dissolve some hydrogen. The hydrogen consumption of an HDT unit can be calculated by eq(11)14.

mH 2  mS  mN  mA  mD  mO

(11)

where mH 2 is the hydrogen flowrate, in Nm3h-1. The superscripts S, N, A, D and O denote desulfurization, denitrogenation, aromatics saturation, dissolution and other hydrogen consumption, respectively. The hydrogen consumption of HDS is shown as below:



Sfeed  Sprod

mS  23vfeed

10000

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(12)

where vfeed is the volumetric flowrate of the feed oil, in m3h-1; Sfeed and Sprod are the sulfur contents in the feed and the product, respectively, in μgg-1. The hydrogen consumption of HDN is written as below:

mN  62vfeed

N feed  N prod 10000

(13)

where N is the nitrogen content, in μgg-1. The hydrogen consumption of HDA can be calculated by eq(14).

mA  480vfeed  Afeed  Aprod 

(14)

where A is the aromatics content, in %. The dissolved hydrogen is in eq(15).

mD  dis  vfeed

(15)

where “dis” is the coefficient of the dissolved hydrogen, in Nm3m-3. In eq(11), the other hydrogen consumption is mainly attributed to olefin saturation and removal of oxygen and metal. Because the olefin saturation is a fast reaction and the oxygen and metal concentrations in streams are much less than other impurities, we assumed that the term mO in eq(11) is constant although the reactions occur under different operating conditions15. Therefore, if the total hydrogen consumption of a certain HDT unit is known, the term mO can be calculated by subtracting the hydrogen consumption of desulfurization, denitrogenation, aromatic saturation and dissolution from


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the total hydrogen consumption. 2.3.2

Pollutant emissions In an HDT unit, the combustion of the fuel gas discharges a substantial amount of

CO2 and a certain amount of SO2. For the fuel products, their combustions emit a certain amount of CO2, SO2 and NOX which can cause damage to the environment. (1) CO2 CO2 is emitted after the combustion of the fuel gas. As for the fuel products, their CO2 emissions are assumed unchanged due to the little variation of the carbon contents in products. Thus, the CO2 emission of the fuel gas is only considered which can be calculated by eq(16).

mCO2 

M CO2 MC

mFGCFG

(16)

where M denotes molecular weight and atom weight, in gmol-1; CFG is the carbon content of fuel gas, in %. (2) SO2 The SO2 emissions of the fuel gas and products are both considered and presented as below:

mSO2 

M SO2 MS

m

FG

S FG +mprod Sprod 

(17)

(3) NOX The calculation of NOX from the combustion of the nitrogen impurities in products is



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shown in eq(18).

mNOX  2.4

M NO2 MN

mprod N prod

(18)

Hydrogenation reaction kinetics The feed oil and hydrogen are reacted under high temperature and pressure to remove

the impurities like sulfur, nitrogen, aromatics, oxygen and metals etc. The reactions of desulfurization, denitrogenation and dearomatization are mainly considered when modeling an HDT process due to the lower contents of oxygen and metals16. The kinetic equations of hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and saturation of aromatics (HDA) are shown as12: (1) HDS kinetics

Sprod

 

S  P H 2  Sfeed exp  kS  LHSV 

   

(19)

(2) HDN kinetics

N prod

 

N  PH 2   N feed exp k N  LHSV 

   

(20)

(3) HDA kinetics

   

 k P A  k M f H2 r XA    A   kr  kf PH 2

A       kf PH 2  kr    1  exp     LHSV    

 


         

(21)

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M

1  Afeed Afeed

(22)

where k denotes the kinetic constant, in h-1; P H 2 is the partial pressure of hydrogen, in MPa; α denotes the pressure dependence term; LHSV is the liquid hourly space velocity, in h-1; X represents the conversion of aromatics, in %; The superscripts f and r are the forward and reverse reactions of HDA. M is the ratio of non-aromatics and aromatics. k  A exp

E R  TR  273.15

(23)

where A is the pre-exponential factor, in h-1; E denotes the activation energy, in Jmol-1; R is the gas mole constant, in Jmol-1K-1; TR denotes the operating temperature, in oC. The pre-exponential factor and active energy of HDS, HDN and HDA are shown in Supporting Information. 2.5

Constraints (1) Operating temperature The operating temperature and pressure would be adjusted when optimizing the

operating conditions. The adjustment ranges should be within the refinery regulations.

TRL  TR  TRU

(24)

where the superscripts L and U are the lower bound and upper bound. (2) Operating pressure

PRL  PR  PRU


(25)


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(3) Impurity contents The optimal impurity contents should not exceed the maximum contents which are specified by environmental standards.

2.6

U Sprod  Sprod

(26)

U N prod  N prod

(27)

U Aprod  Aprod

(28)

Best compromise between economic and environmental objectives For a multi-objective optimization problem, each optimal solution on the Pareto fronts

can be considered as a optimal solution between the economic and environmental objectives. To choose a best compromise from the Pareto fronts, a linear membership function 17 was used to describe weights of the objective functions, which can be expressed as eq(29). The membership function of each optimal solution is shown in eq(30).

ok  Oomax  Ook  Oomax  Oomin 

(29)

 k   o ok

(30)

o

where ok is the kth solution of the oth objective function; Oomax , Oomin and Ook denote the maximum, minimum and the kth values of the oth objective function, respectively;  o is the weight factor of the oth objective function which is optional for decision makers. The best compromise is the one corresponding to the maximum of  k .


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3

Case study

3.1

Original data A cracked diesel HDT unit with annual capacity of 0.6 million tonnes is taken as an

example to illustrate the proposed model which is solved to reduce the operating cost and environmental impacts by optimizing the operating conditions. The related feed properties, operating conditions and their adjustment ranges are shown in Table 1. Table 1 Feed properties, operating conditions and adjustment ranges Operating

Value

Flowrate/ th-1

53.4

Temperature/oC

313

Temperature/oC

280~340

Density/kgm-3

847.7

Pressure/MPa

7.05

Pressure/MPa

6~8

2038/38

LHSV/h-1

2.5

319/12

yRH/%

81.1

S in feed and product/μgg-1 N in feed and product/μgg-1 A in feed and product/%

conditions

Value

Adjustment

Properties

48.7/31.6

ranges

S in product/μgg-1 N in product /μgg-1 A in product/%

Value

Multi-objective Operational Optimization of a Hydrotreating Process

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