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Energy Procedia Procedia 00 139(2017) (2017)000–000 61–66 Energy www.elsevier.com/locate/procedia
International Conference On Materials And Energy 2015, ICOME 15, 19-22 May 2015, Tetouan, Morocco, and the International Conference On Materials And Energy 2016, ICOME 16, 17-20 May 2016, La Rochelle, France The 15th International Symposium on District Heating and Cooling
Modeling the stabilization column in the petroleum refinery Assessing the feasibility of using the heat demand-outdoor Ahmed Ould Brahim, Souad Abderafi*, Tijani Bounahmidi temperature function for a long-term district heat demand forecast LASPI, Mohammadia Engineering School, Mohammed V University in Rabat, Ibn Sina, B.P. 765, Agdal, Rabat, 10090, Morocco
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b
Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France
The objectivecDépartement of this workSystèmes was toÉnergétiques model, the etgasoline stabilization column to4 get a better use of energy, increased Environnement - IMT Atlantique, rue Alfred Kastler, 44300 Nantes, France yield and reduced costs of operation. The calculation of the stabilization column was performed by using Soave-Redlich-Kong (SRK) thermodynamic model with pseudo-component approach. The results given by this model was compared with experimental data obtained from functioning stabilization column of petroleum refinery and conclusions about the accuracy of the models studied Abstract are drawn. Using SRK model, a technical feasibility study was followed to run the stabilization column at an optimum pressure. The results allowed us to highlight the effect of pressure on the separation of products, recommending the optimal pressure for District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the optimizing the energy consumption of reboiler. greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat Due Authors. to the changed climate conditions ©sales. 2017 The Published by Elsevier Ltd. and building renovation policies, heat demand in the future could decrease, prolonging the investment return period. Peer-review under responsibility of the scientific committee of ICOME 2015 and ICOME 2016. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand forecast. Modeling; The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Keywords: simulation; Soave-Redlich-Kong EOS; LPG; Essence; stabilization column; energy buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were with results from a dynamic heat demand model, previously developed and validated by the authors. 1.compared Introduction The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation The crude oil refining produces different chemicals components. Particularly, we distinguish Liquefied Petroleum scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). Gas (LPG) beside the gasoline. The separation of these products necessarily requires a distillation column. This The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the manufacturing oil products, but season its disadvantage energy consumption decrease in the process number is of widely heating used hours to of separate 22-139h during the heating (depending isonthe thehigh combination of weather and [1]. Working between a heat source at the bottom (reboiler) and a cold source on the top (condenser), the distillation renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the column petroleum can be considered as atoheat engine which receives energy the reboiler level and coupledofscenarios). Thefractions values suggested could be used modify the function parameters for theat scenarios considered, and rejects a part in the condenser level,estimations. to separate a liquid mixture [2]. Despite efforts which were conducted by many improve the accuracy of heat demand
researchers to minimize energy consumption, the problem is always posed, to achieve separation. So this research © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +212-537-687-150; fax: +212-537-778-853. Keywords: Heat
[email protected] demand; Forecast; Climate change E-mail address:
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of ICOME 2015 and ICOME 2016. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of ICOME 2015 and ICOME 2016 10.1016/j.egypro.2017.11.173
Ahmed Ould Brahim et al. / Energy Procedia 139 (2017) 61–66 A. Ouel Brahim et al. / Energy Procedia 00 (2017) 000–000
62 2
will firstly devote to model the distillation column of petroleum fractions especially the gasoline stabilization column. Secondly, use the accurate and reliable model to get a better use of energy, increased yield and reduced costs of operation. Nomenclature. Mi: molecular mass of the compound i N: Feed flow rate (kmol/h) Pis Saturation pressure (bar) Q: heat flow (G Cal/h) SG : Specific gravity, at 15°C R: ideal gas constant = 8314.5 J/kmol/K xi: liquid phase mole fraction of component i yi : vapor phase mole fraction of component i zi : mole fraction of component i 2. Industrial Process The petroleum is a complex mixture of different hydrocarbons fractions, small amounts of sulfur and trace amounts of oxygen, nitrogen and metals. Separation is effected by heated the mixture to an elevated temperature of about 400°C, then injected into an atmospheric pressure distillation column. Distillation is the separation of completely miscible mixtures of liquids according to the difference of the boiling point and volatility of the components in the mixture. However, the lighter products, as butane, are obtained at the head of the column and the heavier components such as gasoline, kerosene and gas oil (diesel oil) remains successively lower [3]. As for the residue that cannot be distilled, even at very high temperatures remain at the bottom of the distillation column. Of the tray 13 of distillation column is drawn off the naphtha which feeds stabilization column. The purpose of the latter is to achieve the separation of the total naphtha in its various constituents and prepare the load of catalytic reforming [4]. Stabilization column is equipped with a partial condenser, 30 bubble cap trays and a reboiler (Figure 1). Stabilization column operation is similar to that of the atmospheric distillation column, except that it does not have the side streams. The power supply is disposed at the sixteenth tray (the trays are numbered from top of the column to bottom of the column). The effluent recovered at the column head is the fuel gas. At the first tray level of this column, we obtain the LPG consisting, principally of propane and butane. At the bottom of this column, we get a cut of heavy gasoline containing hydrocarbon chains type C6, C7, C8, C9 and C10 [4]. 2.1. Modeling The set of equations that govern the operation of the column is obtained using the equations of balance material and energy and the equations relating to conditions of Vapor-Liquid Equilibrium (VLE). The figure 1 shows the diagram for principle of an equilibrium stage. Each stage, j receives a diet feed Fj, a fluid flow, Lj-1 from the upper stage and a steam flow, Vj+1 of the lower stage, a liquid extraction Uj, a steam extraction Wj and a heat input Qj can be considered [5]. All these equations required for modeling are written, at each tray of the column and are given below: • Material balance equation is given by:
L j −1 xij −1 − (V j + W j ) yij − ( L j + U j ) xij + V j +1 yij +1Fj zij = 0 • Energy balance equation is given by:
L j −1h j −1 − (V j + W j ) H ij − ( L j + U j ) h j + V j +1hij +1 Fj hFj − Q j = 0 L- h- -V + W H -L + U h +
(1)
Ahmed Ould Brahim et al. / Energy Procedia 139 (2017) 61–66 A. Ouel Brahim et al. / Energy Procedia 00 (2017) 000–000
V h F h -Q = 0
63 3
(2)
Where, V and L are vapor and liquid flow rate, respectively (in kg/h); U and W liquid and vapor side stream flow, respectively (in kg/h). • The heat quantities in the boiler and in the condenser are calculated from the following equations: Q1 = V2 h2 − (U1 + L1 ) h1 − V1h1
(3)
Qn = Vn hn + U n hn − Ln −1hn −1
(4)
Where, Q1 is heat flow of condenser and Qn is heat flow of reboiler.
Figure. 1. (a) Diagram relating to the stabilization column; (b) Principle of an equilibrium stage
• Calculation of VLE: The calculations of VLE are realized by mean of equality of temperature, pressure and fugacity in each liquid and vapor phase. The equation of the latter is written as: f = f fi L = fiV (5)
Where, f and
fi L are fugacities of component, i in vapor and liquid phase, respectively, at the same temperature
and pressure, expressed by: fi L = xiφiL P
(6)
fiV = yiφiV P
(7)
Where, xi and yi are the liquid and vapor phase mole fraction of component and ϕi, is fugacity coefficient. 3. Thermodynamic model VLE calculation requires a thermodynamic model like an equation of state (EOS). Among the many cubic EOS of Van der Waals type currently available, the equation proposed by SRK is widely used due to its simplicity and flexibility, for hydrocarbons fractions [6]. This model has the following form for a pure component [7]:
RT a (T ) P= − V − b V (V − b )
(8)
Where, P is the absolute pressure, T is the absolute temperature, V is the volume and R is the ideal gas constant.
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64 4
The volume and energy parameters of the SRK EOS (a and b), are calculated from the following correlations:
b = 0, 086640. a (T
)=
RTc Pc
( R T c )2
0, 42748
Pc
(9)
(1 + m (1 −
Tr
))
2
(10)
Where, subscripts c and r denote critical and reduced conditions, respectively; m depend on the acentric factor, ω:
m = 0, 48 + 1,57ω + 0,172ω 2
(11)
For extending SRK EOS to mixtures, it is necessary to include composition. Many algebraic relations have been suggested for this purpose. We elected to choose those recommended for SRK EOS [8]. They are the classic Van der Waals one-fluid mixing rules, used to calculated am and bm mixture parameters from:
am =
∑∑x x ( a a ) (1 − k ) 0.5
i
i
bm =
j
i
j
(12)
ij
j
∑x b
(13)
i i
i
kij is the binary interaction parameter between component, i and j; with kij=kji
and
kii=0
3.1. Physicochemical Properties To calculate thermodynamic properties of complex petroleum fluids by using SRK model approximation pseudo component is used. The complex mixture can be regarded as a pseudo-binary liquid mixture, consisting of two pseudo component: LPG and gasoline [9]. To calculate the parameters a, b and m of these two pseudo components, only the critical temperature (Tc), critical pressure (Pc) and acentric factor are needed. These different physicochemical properties can be calculated from the following equations [10]:
Tpc =
1 ∑ zi M iTci ∑ zi M i
Pp c =
1 ∑ z i M i Pc i ∑ zi M i
(14) (15)
ω p = ∑ ziωi
(16)
Where, Tpc, Ppc and ωp are critical temperature, critical pressure and acentric factor of pseudo component. The obtained values for these properties are calculated in this study and summarized in Table 1. Table 1. Critical properties and acentric factors for LPG and gasoline. Pseudocomponent LPG Gasoline
Tpc (K) 398.35 549.09
Ppc(bar) 44.30 49.11
ωpc 11.96 35.11
4. Results and discussion In the first time experimental data are used to test the accuracy of thermodynamic simulation model. These data were collected from a sample of the stabilization column (Table 2). The feed of this unit derived from the atmospheric distillation is composed of traces of H2 and H2S, small amounts of the hydrocarbon type C1, C2, C3, C4 and C5, as well as a heavy gasoline fraction containing hydrocarbon chains of type C6, C7, C8, C9, C10 and traces of
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C11. The Fuel Gas is recovered at the top of the column. At the first tray, the LPG is obtained, at a pressure equal to 13.7 bar. The stabilized gasoline is recovered at the bottom column, at a pressure equal to 14.1 bar. Using the SRK EOS with kij=0 and with the help of the Pro II, process simulation software, the calculated results are compared to the experimental data by calculating the error thanks to the following relationship: E (%) =
Ve − Vc Ve
(17)
× 100
Where, Vc and Ve are the calculated and the experimental values, respectively. This comparison allowed us that the results are satisfactory if we consider the experimental error. Table.2. Experimental data for stabilization column. Compound H2 H2S C1 C2 C3 i C4 n C4 i C5 n C5 C6 C7 C8 C9 C10 C11 N (kmol/h) SG P (bar) Te (°C)
Zi Gasoline
Alimentation 6,97E-02 2,56E-03 3,35E-02 2,13E-02 3,20E-02 2,90E-02 6,49E-02 7,46E-02 7,69E-02 1,49E-01 1,45E-01 1,48E-01 7,75E-02 7,34E-02 1,99E-03 84716.09 0.65 19.8 132
LPG 0,10137 0,02816 0,08873 0,08661 0,15846 0,15309 0,34013 0,03469 0,00876 0,00001
0,00042 0,00135 0,09218 0,10214 0,20160 0,19591 0,19985 0,10469 0,09916 0,00269 65371.6 0.74 14.1 122.5
10964.3 0.60 13.7 50
In the second time, the parametric sensitivity study was tested for pressure, at the top of the column. By choosing different pressure values below 13 bar, we had tried to obtain the experimental data related to the boiling temperature, mol flow rates and the specific gravity (SG), at 15°C of LPG and gasoline. The result allowed us to conclude that the minimum pressure required at the top of the stabilization column is 9 bar. The simulation results given for this pressure are summarized in the Table 3. Table 3. Simulation results at 9 bar. LPG
Gasoline Vc E(%) 122.68 0.15
Te (°C)
Vc 51.89
E(%) 3.78
N (Kmol/ h)
10963
0.07
65332.08
0.06
SG
0.56
6.67
0.70
5.41
Calculating heat flow, of condenser and of reboiler, were also performed for different pressures studied. The results obtained are grouped in Table 4, which shows that for the use of a pressure equal to 9 bar, the heat flow of the reboiler decreases, which enables us to save 1.2 Gcal / h. The decrease pressure at the top of column leads to the lowering of the circulation of the vapor and liquid in the trays. The reduction of the vapor-liquid circulation in the distillation column is the reduction of the irreversibility of distillation system. The contribution to the irreversibility is mainly due to thermal effects in the reboiler and in the condenser, which explains the decrease in energy consumption of the reboiler [11].
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A. Ouel Brahim et al. / Energy Procedia 00 (2017) 000–000 Ahmed Ould Brahim et al. / Energy Procedia 139 (2017) 61–66 Table 4. Comparison of Heat Flux. P (bar)
Condenser (Gcal/h)
Reboiler (G cal/h)
13.7
1.8
5
9
1.8
3.8
5. Conclusions In the present study, attention is given to get a better use of energy for the distillation column of petroleum fractions, especially the gasoline stabilization column. In a first time, equations to modeling this column, in the oil process are described, for each equilibrium stage. These equations are based on the balance of material and energy and conditions of VLE. The SRK EOS was used to calculate thermodynamic properties of complex petroleum fluids, using the pseudocomponent approach. The naphtha which feeds stabilization column was considered as a pseudo-binary liquid mixture, consisting of two pseudocomponent: LPG and gasoline. The accuracy of the model was tested successfully, by comparing the prediction of the boiling temperature, mol flow rates and the specific gravity at 15°C of LPG and gasoline, to corresponding experimental data obtained from functioning crude oil distillation column of Moroccan refinery. In the second time, a technical feasibility study was followed to run the stabilization column at an optimum pressure. The optimized value of pressure and that usually applied in refinery were used to calculate heat flow of condenser and of reboiler. The results allowed us to high light the effect of pressure on the separation of products. From an economic point of view, stabilization with optimal pressure may contribute to a decrease in expenses related to the energy consumption of reboiler.
Acknowledgements The authors gratefully acknowledge support provided by Moroccan Petroleum refinery SAMIR (Société Anonyme Marocaine de l’Industrie du Raffinage).
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