Article pubs.acs.org/IECR
Adsorption of Polycyclic Aromatic Hydrocarbons from Heavy Naphthenic Oil Using Commercial Activated Carbons. 2. Column Adsorption Studies F. Murilo T. Luna, A. Nilson Oliveira Filho, Caio C. B. Araújo, Diana C. S. Azevedo, and Celio L Cavalcante, Jr.* Departamento de Engenharia Química, Grupo de Pesquisa em Separações por Adsorçaõ , Núcleo de Pesquisas em Lubrificantes, Universidade Federal do Ceara, Campus do Pici, Bl. 709, Fortaleza, CE 60.455-900, Brazil ABSTRACT: Continuous adsorption and desorption of polycyclic aromatic hydrocarbons (PAHs) from heavy naphthenic oils (HNOs) were performed using commercial activated carbon on fixed bed experiments. In part 1, HNO samples with high PAH content (ca. 8 wt %) were treated in batch adsorption experiments, showing decrease in PAH concentration to ca. 3%. In this study, successive column experiments were carried out to study the reduction in capacity along continuous cycles and optimal operational conditions. A column simulation model, using the particle parameters obtained in the previous study, was applied to predict breakthrough curves for adsorption and desorption runs. After several cycles of adsorption/ desorption, using solvent or inert gas, only a slight decrease in PAH adsorption capacity was observed. Selectivities for PAH adsorption in relation to aromatics adsorption were also estimated from the adsorption breakthrough curves data. A conceptual continuous process was simulated using the proposed model and evaluated based on productivity in terms of volume of treated oil per mass of adsorbent per time, showing a maximum value at space velocity of ca. 4 h−1.
1. INTRODUCTION The behavior of activated carbons and acid clays for total aromatics removal from light mineral naphthenic oil (MNO) was investigated by Luna et al.,1 with activated carbons being more adequate for aromatics removal in that system. However, the removal of polycyclic aromatic hydrocarbons (PAH) from complex oil mixtures has been scarcely reported in the open literature. In Part 1 of this study, Luna et al.2 reported equilibrium and kinetic data for the batch adsorption of polycyclic aromatic hydrocarbons (PAH) from heavy naphthenic oil (HNO) using commercial activated carbons. In the present article, the same system (PAH/HNO) will be evaluated in a continuous operation mode using a fixed bed adsorption column experimental setup. Studies focusing on removal of PAH from hot gas emissions3,4 and contaminated soils5−7 have been performed using adsorption procedures. Several materials have been reported for PAH removal in different systems, such as plant residues,8,9 low-cost adsorbents of natural origin,10 mesoporous organosilica,11 coke-derived porous carbon,12 mesoporous materials,13 and commercial activated carbons.1,2,14 Adsorption appears as a promising process because of its low-energy demand, possibility of adsorbent regeneration and broad availability of adsorbents.15 In this study, samples of heavy naphthenic oil (HNO) was treated for PAH removal in continuous mode using adsorption onto granular commercial activated carbons in a fixed bed © XXXX American Chemical Society
experimental setup. HNO is a vacuum industrial distillate with viscosities varying between 380 and 420 cSt (at 40 °C), obtained from different types of Brazilian crude petroleum and mainly applied to formulation of special waxes and lubricants. Without proper treatment, HNO presents high PAH concentration. Hence, it is important to reduce its concentration, to encompass regulatory requisitions for commercial applications. Oils with PAH contents lower than 3 wt % are classified as noncarcinogenic.16 A column simulation model, including a dual-resistance (pore and surface) particle diffusion model, was applied to interpret and validate the experimental results. Parameters such as adsorption capacity in adsorption and desorption breakthrough steps, adsorbent regenerability after up to 10 adsorption/desorption cycles, and selectivities for PAH adsorption in relation to other aromatics were used to evaluate the dynamics behavior of the adsorption column. Productivity in terms of volume of treated HNO (≤3 wt % of PAH) per mass of adsorbent per time, in a conceptual continuous process, was also proposed and evaluated against the space velocity in the column. Received: April 18, 2016 Revised: June 28, 2016 Accepted: July 1, 2016
A
DOI: 10.1021/acs.iecr.6b01492 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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2. EXPERIMENT AND MODELING 2.1. Materials. The HNO sample was provided by PETROBRAS (Brazil). Dimethyl sulfoxide, DMSO (>99 wt %, JT Baker, USA), spectrophotometric grade cyclohexane (Merck, USA), sodium chloride (JT Baker, USA), and deionized water obtained by the Milli-Q system (Millipore, USA) were used in procedures of polycyclic aromatic extractions. Solutions of naphthalene (>98 wt %, Acros Organics, USA) in cyclohexane at different concentrations were used for calibration of the total aromatics method. nHexane (>99 wt %, JT Baker, USA) was used as solvent in the column experiments. Commercial nitrogen (>90 wt %) was provided by White Martins-Praxair, Brazil. A granular activated carbon, provided by Norit (Netherlands), was used in this study (GAC 830W). The properties (PSD and textural parameters) and the treatment of the adsorbent prior to the column experiments have already been reported in Part 1 of this study.2 2.2. Column Experiments. Adsorption and desorption curves of HNO through a fixed bed system (Figure 1) were
Following the adsorption experiments, desorption procedures were carried out. For this, the experiment has been performed with the same operating conditions and sampling procedures, passing pure solvent (n-hexane) through the column. For the desorption step, the area under the curve provides a confirmation of the aromatics or PAH amounts retained in the column during the adsorption step. The regenerability of the bed after multiple cycles was evaluated using two different procedures: initially using only solvent at the same operational conditions; and afterward using inert gas (N2) at different temperatures (120−150 °C). 2.3. Modeling of the Column Adsorption/Desorption Process. A general rate model, using pore and surface diffusion resistances in the particle18−20 was applied to predict the breakthrough curves and validate the experimental data (eqs 2 to 7): Differential mass balance in liquid phase ε
3k ∂C ∂C ∂ 2C +u = εDL 2 − (1 − ε) f (C − Cp|r = R p ) R ∂t ∂z ∂z (2)
Initial conditions t = 0,
C(z , 0) = 0;
Cp(z , 0) = 0;
q(z , 0) = 0 (3)
Boundary 1 z = 0,
⎡ ∂C ⎤ ⎢⎣uC − DL ⎥⎦ = uC0 ∂z
(4)
∂C =0 ∂z
(5)
Boundary 2 z = L,
Differential mass balance in solid phase Figure 1. Experimental setup used for adsorption/desorption column experiments.
εp
obtained at different conditions. The adsorbent was packed into an adsorption column which was connected to a pumping system (Varian ProStar 210, USA). Initially, pure solvent (nhexane) was delivered by the pump in order to adjust the flow rates through the bed. Following this, the HNO samples (without solvent) were injected into the column and samples were taken periodically at the outlet and analyzed. The total aromatic and PAH concentrations were obtained using Fourier transform infrared (FTIR) methods, as previously reported in Luna et al.17 All column experiments were carried out at 30 °C. The adsorption capacity was calculated using the breakthrough curve for each experiment. The area above the curve corresponds to the integral which appears in the column massbalance (eq 1) to calculate the amount adsorbed in equilibrium (q0) with initial concentration (C0). q0 =
C0 ⎡ ⎢Q MB ⎢⎣
∫0
t
⎤ ⎛ C⎞ ⎜1 − ⎟ dt − VCε⎥ ⎥⎦ C0 ⎠ ⎝
∂Cp
∂q ρ ∂t ∂t ap ρap ∂ ⎛ ∂q ⎞ 1 ∂ ⎛ ∂Cp ⎞ = Dp 2 ⎜r 2 ⎟ + DS 2 ⎜r 2 ⎟ r ∂r ⎝ ∂r ⎠ r ∂r ⎝ ∂r ⎠ + (1 − εp)
(6)
Boundary 1 r = 0,
∂Cp ∂r
=0
(7)
Boundary 2 r = R p,
Dp
∂Cp ∂r
+ DS
∂q ρ = k f (C − Cp) ∂r a p
(8)
Instantaneous equilibrium is assumed between the concentration in the fluid phase within the particles (Cp) and the concentration in the solid adsorbent (q), for any particle radius (r), eq 9. q = f (Cp)
(9)
where Cp is the intraparticle liquid phase concentration and q is a function of Cp calculated from the Toth equation, obtained through batch experiments (see Luna et al.2). C is the concentration in bulk (mg/g), DL is the axial dispersion coefficient (cm2/min), kf is the mass transfer coefficient on the film (cm/min), Dp is the pore diffusion coefficient (cm2/min),
(1)
where q0 is the solid phase concentration (mg/g of adsorbent), C0 is the feed concentration (mg/g), C is the concentration in bulk (mg/g), MB is the bed mass (g), VC is the column volume (cm3), and ε is the bed porosity. B
DOI: 10.1021/acs.iecr.6b01492 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research DS is the surface diffusion coefficient (cm2/min), u is the superficial velocity (cm/min), Rp is the average particle radius (cm), r is the radial coordinate, ε is the bed porosity, z and t are the spatial coordinates (cm) and time (min), respectively. The axial dispersion coefficients were estimated using eq 10.21 ε
DL = 0.2 + 0.011Re 0.48 u2R p
operational conditions and later using inert gas at high temperatures. 3.1. Assessment of Adsorption Steps. The experimental data of the HNO adsorption experiments with GAC at 30 °C are shown in Figure 2. Adsorption breakthrough curves in
(10)
The external mass transfer coefficient was calculated from the following correlation proposed by Wakao and Funazkri:22 Sh ≡
2R pk f Dm
= 2 + 1.1(Sc)1/3 (Re)0.6
(11) 2
The molecular diffusivity (Dm), in cm /min, of PAHs was estimated based on the Wilke and Chang correlation (Poling et al.23): Dm = 4.44·10−6
(ϕ MM)1/2 T ηV b0.6
(12)
where MM is the molar mass of HNO, η is the oil viscosity, T is the temperature, Vb is the molar volume of the diffusing molecule at the boiling temperature, and ϕ is an association coefficient, assumed to be 1.0 for aromatics. The value of Vb (213.8 cm3/mol) was obtained by LeBas volumes as described in Poling et al.,23 using pyrene as a model molecule. The pore diffusion coefficient was estimated from the molecular diffusivity (eq 13), using tortuosity factor of 5.0, as usually found for this type of system.24,25 Dp =
Dmεp (13)
τ
The surface diffusion coefficient (DS) was obtained from the particle adsorption studies previously reported in Luna et al.2 For the desorption simulations, the same mass balances are used, with the following initial and boundary conditions for the column balance: Initial conditions t = 0,
C(z , 0) = C0
t = 0,
Cp(z , 0) = C0 ;
Figure 2. Breakthrough curves in terms of (a) total aromatics (C0 = 217 mg/g) and (b) PAH (C0 = 73 mg/g) of HNO samples over GAC at 30 °C: ε = 0.39; Q = 0.20 mL/min: (□) 1st cycle; (○) 2nd cycle; (×) 3rd cycle.
terms of total aromatics (Figure 2a) and polyaromatics hydrocarbons (Figure 2b) are shown for three consecutive adsorption steps. According to the areas above the curves, adsorption capacities calculated using eq 1 were estimated (see Table 1).
(14)
q(z , 0) = f (Cp),
∀r (15)
Boundary in liquid phase z = 0,
⎡ ∂C ⎤ uC − DL ⎥ = 0 ⎣⎢ ∂z ⎦
Table 1. Adsorption Capacities and Equilibrium Selectivities for Three Successive Cycles (16)
adsorption capacity (mg/g of ads)
The system of algebraic and partial differential equations, with the respective initial and boundary conditions, was implemented according to the standard gPROMS syntax.26 Discretization in the axial and radial domains was performed using a third order orthogonal collocation method in finite elements (OCFEM) with six sections and three placing spots per section.
cycle
total aromatics
PAH
αPAH/A
1st 2nd 3rd
218.0 199.1 197.3
184.8 178.2 175.8
10.9 16.8 16.1
It may be observed that the GAC showed initially adsorption capacities of 218.0 and 184.8 mg/g assessed in terms of total aromatics (Figure 2a) and PAHs (Figure 2b), respectively. These values are in good agreement with the values obtained for the same activated carbon sample in the particle batch experiments previously reported (Luna et al.2). Changes in adsorption capacities after two and three cycles may also be seen in Table 1, showing slight reduction in both total
3. RESULTS AND DISCUSSION Adsorption and desorption cycles were carried out to assess operation conditions of using activated carbon (GAC) to remove PAHs from heavy naphthenic oil in a continuous column process. The regenerability of the bed after each cycle was evaluated initially by using only solvent at the same C
DOI: 10.1021/acs.iecr.6b01492 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research aromatics and PAH capacities with the number of cycles. However, selectivity values between PAH and single aromatics (A), estimated using eq 17 (Luna et al.2), increase with the number of cycles from 11.0 at the first cycle to around 16 after the second and third cycles. Again there is an agreement between the αPAH/A estimated for the first cycle in the column experiment with the value obtained previously in the batch experiments (ca. 10.1). αPAH/A =
qPAH /(qTA − qPAH) C PAH/(C TA − C PAH)
(17)
The column simulation model shown in eqs 2−12 was evaluated with the experimental data obtained in the first cycle with input parameters shown in Table 2. A comparison Table 2. Input Parameters for the Column Simulation Model model parameters
values
Rp εp L D Q C0 ε DL kf Dm tortuosity DS
0.03 0.47 25.0 0.46 0.2 73 0.39 2.52 0.094 2.77·10−5 5.0 2.10·10−9
units cm cm cm cm3/min mg/g cm2/min cm/min cm2/min
Figure 4. Desorption curves for HNO over GAC at 30 °C using solvent at same adsorption conditions (ε = 0.39; Q = 0.20 mL/min): (a) 1st cycle and model simulation results; (b) data for three successive adsorption/desorption cycles and respective adsorbent capacities.
cm2/min
between the experimental breakthrough curve and the simulated data is shown in Figure 3. A remarkable agreement
capacity of 180.7 mg/g can be estimated from this first desorption run, which is slightly lower than the previously estimated value in the first adsorption step (see Table 1). This suggests that some compounds were retained in the adsorbent and are not completely desorbed using only the solvent. The same simulation model, with proper modifications in initial and boundary conditions (eqs 14−16), was evaluated with the data of the desorption step. It may be observed that, using again only the data obtained from the batch experiments, operational conditions, and column and oil properties, there is a remarkable good representation of the experimental desorption curve (Figure 4a). The experimental data of three adsorption/desorption cycles using n-hexane as solvent at the same temperature and flow rate are shown in Figure 4b. A slight decrease (ca. 6%) in adsorbent capacity with the number of cycles may be observed (180.7 mg/g for the first cycle to 170.0 mg/g for the third cycle) from this data. Since the adsorption capacity of the adsorbent in the bed decreases slightly with the number of cycles, suggesting that some compounds may be retained in the adsorbent even after the liquid solvent elution, further experiments were planned to more fully desorb the PAHs from the column, by raising the bed temperature under a flow of inert gas. For this purpose, additional adsorption/desorption cycles, starting again with fresh adsorbent in the column, were performed to evaluate the trend in adsorbent capacity reduction with respect to the duration and temperature of the desorption step with inert gas.
Figure 3. Breakthrough curve in terms of PAH (C0 = 73 mg/g) of HNO samples over GAC at 30 °C, Q = 0.20 mL/min: (□) experimental data; full line is the model representation.
may be observed, especially because this simulation was performed using solely the data obtained from the batch experiments (Luna et al.2), operational conditions, and column and oil properties. 3.2. Evaluation of the Adsorbent Regenerability. Adsorption/desorption cycles were performed to assess the regeneration of the adsorbent in a continuous process. The experimental data of the first desorption step with solvent at the same temperature of 30 °C, in which the adsorption was previously assessed, are shown in Figure 4a. An adsorption D
DOI: 10.1021/acs.iecr.6b01492 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research After the bed saturation, the temperature of the system was raised, as shown in Table 3, for 10 consecutive cycles, with a gas Table 3. Experimental Conditions of 10 Adsorption/ Desorption Cycles Using Hot Inert Gas in the Desorption Steps run
desorption time
temperatures
adsorption capacity (mg/g)
1 2 3 4 5 6 7 8 9 10
60 min
120 °C
182.7 180.5 176.2 173.4 171.3 170.9 170.2 169.4 169.5 169.4
120 min
180 min
150 °C
flow of N2 of 40 N mL/min at the column inlet. The breakthrough curves of the adsorption steps in cycles 1, 5, and 1 are shown in Figure 5, and very similar curves were obtained
Figure 6. Conceptual continuous process using two columns operated simultaneously in adsorption/desorption cycles.
subsequent cycles. Simulations were then performed at different space velocities in the adsorption step, calculating the switching time necessary to reach the desired purity of treated HNO in the product tank, and thus estimating a productivity in terms of volume of treated oil per mass of adsorbent per time (eq 18). ts
productivity =
F(1 − C tr)∫ (1 − C) dt 0
MB(1 − C0)ts
(18)
where F is the feed flow rate, C0 is the feed concentration, C is the concentration in the liquid phase as a function of time, Ctr is the concentration of treated oil (3.0 wt %), MB is the adsorbent mass, and ts is the switching time. As observed in Figure 7, the
Figure 5. Adsorption runs in terms of PAH (C0 = 73 mg/g) over GAC at 30 °C and feed flow rate of 0.20 mL/min: (□) 1st cycle; (○) 5th cycle; (×) 10th cycle. Adsorbent regenerated between cycles by raising the temperature (120−150 °C) using inert gas.
even after 10 cycles. An estimate of adsorbent capacity with the number of cycles is also reported in Table 3. It may be observed that the capacity is kept essentially constant after the fifth cycle, an interesting feature for a continuous operation process. 3.3. Evaluation of Operational Conditions in a Continuous Process. Following the modeling and understanding of the phenomena involved in a column adsorption process of PAHs in activated carbons, a possible industrial continuous process is proposed schematically in Figure 6. Two columns with adsorbent may be operated simultaneously in adsorption/desorption cycles. While column A is adsorbing the PAHs from a HNO feed (7.3 wt % PAHs) at 30 °C at a given space velocity (ν), column B is being regenerated, using inert gas at high temperature, desorbing the material adsorbed in the previous step. This adsorption step is run through column A until a product of treated HNO with 3 wt % of PAHs is obtained in the tank. At this point of time (ts), the feed is switched to column B, and desorption starts from column A. For the sake of evaluating different operational conditions, we assumed that the parameters obtained from the first experimental adsorption cycle would be reproducible in
Figure 7. Productivity in a continuous process, in terms of (m3 of oil) per (mass of adsorbent) per (hour) of treated HNO with PAHs concentration less than 3 wt %, against space velocity (h−1).
productivity increases with increasing space velocity up to values of ν of ca. 4.0 h−1. For higher values of space velocity, the mass transfer resistances hinder adsorption of PAH within the particle and the productivity decreases with increasing space velocities.
4. CONCLUSIONS Continuous adsorption and desorption of polycyclic aromatic hydrocarbons (PAHs) from heavy naphthenic oils (HNOs) E
DOI: 10.1021/acs.iecr.6b01492 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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qPAH = concentration of pAHs in the solid phase (mg·g−1) qTA = concentration of total aromatics in the solid phase (mg·g−1) r = radial coordinate (cm) Rp = average particle radius (cm) Re = dimensionless Reynolds number Sh = dimensionless Sherwood number Sc = dimensionless Schmidt number t = time (min) ts = switching time in conceptual cyclic process (min) T = absolute temperature (K) u = superficial velocity (cm·min−1) Vb = molar volume at normal boiling point (cm3·g mol−1) VC = column volume (cm3) z = axial coordinate (cm)
were studied using commercial granular activated carbon (GAC) in fixed bed column experiments. A general rate model, including pore and surface diffusion resistances in the particle, was applied to simulate the behavior for adsorption and desorption runs. Successive column experiments were carried out to study the reduction in capacity along continuous cycles using solvent or inert gas. An adsorption capacity of 185 mg/g, assessed in terms of PAHs, was obtained for the fresh activated carbon sample. A slight decrease (ca. 6%) in adsorbent capacity was observed after three adsorption/ desorption cycles using n-hexane as solvent in the desorption step. For desorption with inert gas at high temperatures, the capacity was kept essentially constant after the fifth cycle, an interesting characteristic for a continuous operation process. Finally, a continuous process was evaluated for a proposed setup with two columns operated simultaneously in adsorption/desorption cycles. Productivity values, expressed in terms of volume of treated HNO (≤3 wt % of PAH) per mass of adsorbent per time, increased with increasing space velocity up to values of ca. 4.0 h−1. For higher values of space velocity (ν greater than 4 h−1), the mass transfer resistances apparently hinder adsorption of PAH within the particle and the productivity decreased, showing thus an optimal value of space velocity to be used in that conceptual process.
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Greek Symbols
AUTHOR INFORMATION
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Corresponding Author
*Tel.: +55-85-3366-9611. Fax: +55-85-3366-9601. E-mail:
[email protected].
αPAH/A = selectivity of pAH with respect to single aromatics compounds ε = bed porosity εp = particle void fraction η = viscosity (cP) ϕ = association coefficient ραp = particle apparent density (g·cm−3) τ = tortuosity factor ν = space velocity (h−1)
REFERENCES
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Present Address #
C.C.B.A.: IFP Training Middle East, Manama Centre, Entrance 4, Office 506, Diplomatic Area, Manama, PO Box 10813, Kingdom of Bahrain. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors wish to thank financial and logistic support provided by PETROBRAS (Petróleo Brasileiro S.A.) and CNPq (Conselho Nacional de Pesquisa e Desenvolvimento ́ Cientifico). The authors are also grateful to NORIT Activated Carbon (Netherlands) for providing activated carbon samples.
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SYMBOLS C = bulk liquid phase concentration (mg·g−1) C0 = feed concentration (mg·g−1) Cp = intraparticle liquid phase concentration (mg·g−1) CPAH = concentration of pAHs in equilibrium (mg·g−1) CTA = concentration of total aromatics in equilibrium (mg· g−1) Ctr = concentration of treated oil ( wt %) DL = coefficient of axial dispersion (cm2·min−1) Dm = molecular diffusion coefficient (cm2·min−1) Dp = pore diffusion coefficient (cm2·min−1) Ds = surface diffusion coefficient (cm2·min−1) F = feed flow rate (m3·h−1) kf = film mass transfer coefficient (cm·min−1) MM = molecular weight (g·mol−1) MB = mass of the adsorbent (g) Pe = dimensionless Péclet number Q = flow-rate (mL·min−1) q = solid phase concentration (mg·g−1) F
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