Extraction of aromatics from petroleum naphtha reformate by solvent.pdf

8/10/2019 Extraction of aromatics from petroleum naphtha reformate by solvent.pdf

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controlled within 0.1 K. In all conditions, a stirring period of 1 hand a settling time of 24 h were allowed. No sensible changes inthe equilibrium compositions occurred at longer extraction or set-tling times.

The miscibility temperature (Tm) of the mixture was measuredby adding equal volumes of solvent and reformate and mixingthe two phases completely. A magnetic stirrer/heater was usedto raise the mixture temperature gradually. The miscibility tem-perature was marked as the point where the turbidity of the mix-ture disappears and the two phases become completely miscible ineach other.

The equilibrium concentrations of all components in each phasewere finally measured using a Varian CP3800 gas chromatographequipped with flame ionization detector (FID) and a capillary col-umn 50 m 0.25 mm i.d. CP-Sil PONA CB Fused Silica WCOTDF = 0.5lm. The FID detector and the injection port were main-tained at T =554 K. The oven temperature was first kept atT =308 K for 20 min and then raised to 523 K with the rate of3 K min1, holding this temperature for 10 min.

The experimental temperature uncertainty is expected to beless than 0.2 K. The accuracy of the measured compositions is ex-

pected to be better than 2%.

3. Modelling

The use of UNIFAC model[6]to predict the performance of themixed solvent, ethylene carbonate and 1-cyclohexyl-2-pyrroli-done, in the selective extraction of aromatics from naphtha refor-mate, requires knowledge of the group volume and surface areaparameters (R and Qvalues) and the interaction parameters ofthe group-pairs present in the system. The groups, which are usedin this study are CH3, CH2, CH, C, ACH, ACCH2, ACCH3, ethylene car-bonate (EC) and 1-cyclohexyl-2-pyrrolidone (CHP) (the polar nat-ure of EC and CHP results in their treatment as single groups).

Consequently, 9 groups and 24 group-pairs are detected. The RandQ values of the sub-groups CH3, CH2, CH, ACH, ACCH3 and

ACCH2are taken from Ref. [7], and those of EC and CHP are selectedfrom[5]. The group interaction parameters reported in Ref.[3,5]are applied in this work.Tables 13summarize the UNIFAC model[6]groups and parameters[3,5,7].

4. Results and discussion

The predictions of the phase equilibria have shown some devi-

ations from experimental results when treating small concentra-tions such as those of the aromatics in the extract phase and ECand CHP in the raffinate phase. The percent root mean square devi-ation (RMSD) has been used to assess these deviations. The (RMSD)value is defined as follows[3]:

%RMSD 100X

XEi;exp XEi;calc

2

XXRi;exp X

Ei;calc

2h i

=2Nn o1

2

;

1

where Nis the number of components in the system,Xi,expandXi,calcare the experimental and calculated mass fractions and the super-scripts E and R refer to the extract and raffinate phases, respectively.TheRMSDvalues and the experimental compositions are reportedintables 4 to 9. The results ofRMSDcalculations (an average value

equal to 4.86%) indicate acceptable agreement between the pre-dicted values and the obtained experimental data.

4.1. Optimization of solvent to feed ratio

Figure 2shows the aromatic content in extract phase at varioustemperatures for different solvent compositions. As can be seen,the maximum separation can be done with 40% CHP. The optimumvalue of the solvent to feed ratio for this solvent composition is alsocalculated.

Two independent parameters of the multi component systemsextraction at constant pressure are extraction temperature (T)and solvent to feed ratio (S/F). It is recommended by Singh[8]tocombine these independent parameters into a dimensionless

parametere.g.(Tm/T). (The values ofTmfor (S/F= 1) ratio for a sol-vent with 40% CHP is expected to be 351 K).

Thus, it is interpreted that the dimensionless parameter(Tm/T)(S/F) leads to a better description of multi-componentsystems at various temperatures and varying in the value of thesolvent to feed ratios. In this work, this parameter is referred asthe operation factor (OF).

TABLE 2

Rand Qvalues for the UNIFAC groups[5,7].

Group R Q Group R Q

CH3 0.9011 0.848 ACCH2 1.0396 0.660CH2 0.6744 0.450 ACCH3 1.2663 0.968CH 0.4469 0.228 EC 2.9727 2.520

ACH 0.5313 0.400 CHP 6.8987 2.580

TABLE 1

Reformate composition and the number of functional UNIFAC groups in each of its constituents [5].

No Component Mass/% No. of UNIFAC groups in each component

CH3 CH2 CH C ACH ACCH3 ACCH2

1 n-Hexane 7.35 2 4 0 0 0 0 02 Cyclohexane 2.48 0 6 0 0 0 0 03 Benzene 2.96 0 0 0 0 6 0 04 n-Heptane 9.86 2 5 0 0 0 0 05 Toluene 21.7 0 0 0 0 5 1 06 Iso-octane 12.2 5 1 1 1 0 0 07 Ethylbenzene 6.33 1 0 0 0 0 0 18 Xylene 37.1 0 0 0 0 4 2 0

FIGURE 1. Schematic diagram of experimental setup.

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For each run, certain desirable dependent properties namely thesolvent capacity, solvent selectivity, solvent power and to-tal aromatic content have been calculated as follows:

The solvent capacity (C) is defined as the ratio of the sum of themole fractions of all of the aromatic components in the extract, xEA,to that in the raffinate,xRA [9]:

CxEAxRA

: 2

The selectivity (S) for the solvent is defined by the following

relation[9]:

SxEA=xRA

xEP=xRP

; 3

wherexEPandxRPare sum of the mole fractions of the paraffinic com-

ponents in the extract and raffinate phases, respectively.The solvent power (P), which is a measure of the fraction of

hydrocarbons in the extract, is calculated using the following equa-tion[10]:

P xEA xEP: 4

TABLE 5Predicted (liquid + liquid) equilibria of the petroleum naphtha reformate with 35% CHP in solvent.

Component T= 300K T= 310K T= 320 K T= 330 K

XR XE XR XE XR XE XR XE

n-Hexane 0.0209 0.0014 0.0228 0.0012 0.0222 0.0019 0.0222 0.0019Cyclohexane 0.0465 0.0034 0.0505 0.0032 0.0493 0.0047 0.0484 0.0053Benzene 0.0198 0.0132 0.0238 0.0107 0.0217 0.0124 0.0227 0.0117n-Heptane 0.0731 0.0062 0.0809 0.0047 0.0773 0.0083 0.0783 0.0075Toluene 0.2561 0.1014 0.2713 0.0998 0.2595 0.1106 0.2652 0.1063Iso-octane 0.036 0.003 0.0407 0.0016 0.0393 0.0031 0.0395 0.0029Ethylbenzene 0.0414 0.0151 0.0432 0.0155 0.0411 0.0173 0.0427 0.0162Xylene 0.1494 0.0508 0.1572 0.0513 0.1537 0.0552 0.1557 0.0537EC 0.1023 0.5583 0.0786 0.5473 0.1255 0.5077 0.1064 0.5219CHP 0.2543 0.2472 0.2309 0.2647 0.2102 0.2789 0.2189 0.2728%RMSD 3.96 4.06 4.66 4.13

TABLE 4

Experimental (liquid + liquid) equilibria of the petroleum naphtha reformate with 35% CHP in solvent.

Component T= 300K T= 310K T= 320K T= 330 K


n-Hexane 0.03 0.002 0.038 0.002 0.035 0.003 0.035 0.003Cyclohexane 0.068 0.005 0.078 0.005 0.073 0.007 0.073 0.008Benzene 0.027 0.018 0.038 0.017 0.035 0.02 0.035 0.018n-Heptane 0.083 0.007 0.087 0.005 0.084 0.009 0.084 0.008Toluene 0.293 0.116 0.299 0.11 0.284 0.121 0.287 0.115Iso-octane 0.048 0.004 0.051 0.002 0.051 0.004 0.054 0.004Ethylbenzene 0.041 0.015 0.039 0.014 0.038 0.016 0.037 0.014Xylene 0.15 0.051 0.147 0.048 0.142 0.051 0.145 0.05EC 0.118 0.644 0.093 0.648 0.157 0.635 0.128 0.628CHP 0.142 0.138 0.13 0.149 0.101 0.134 0.122 0.152

TABLE 6


Component T= 300K T= 310K T= 320K T= 330 K


n-Hexane 0.022 0.003 0.029 0.006 0.027 0.007 0.026 0.009Cyclohexane 0.053 0.009 0.058 0.012 0.062 0.013 0.063 0.017Benzene 0.024 0.02 0.038 0.023 0.033 0.024 0.029 0.023n-Heptane 0.071 0.011 0.077 0.011 0.074 0.013 0.072 0.012Toluene 0.275 0.148 0.283 0.139 0.275 0.159 0.275 0.155Iso-octane 0.048 0.008 0.053 0.005 0.056 0.007 0.06 0.006Ethylbenzene 0.041 0.02 0.038 0.018 0.037 0.019 0.035 0.019Xylene 0.15 0.072 0.144 0.069 0.141 0.075 0.143 0.071EC 0.157 0.546 0.128 0.548 0.158 0.52 0.152 0.507

CHP 0.159 0.163 0.152 0.169 0.137 0.163 0.145 0.181

TABLE 3

UNIFAC interaction parameters according to the equation aij a0ij a

1ijT=K 273:15

[3,5].

i j a0ij/K a1ij a

ij0/K a

ij1

EC CHP 243.891 4.314 244.837 1.738EC CH3,CH2,CH 318.935 10.072 449.929 1.869EC ACH 507.630 9.684 2870.330 27.379EC ACCH2, ACCH3 2324.10 49.610 2529.380 26.943

CHP CH3,CH2,CH 95.834 2.763 1142.260 24.753CHP ACH 191.253 4.920 1451.600 31.520CHP ACCH2, ACCH3 659.900 14.985 249.215 5.358CH3,CH2,CH ACH 1274.86 30.054 707.591 24.061CH3,CH2,CH ACCH2, ACCH3 311.725 7.206 738.353 22.249ACH ACCH2, ACCH3 657.130 17.963 265.015 8.753

32 A. Fazlali et al. / J. Chem. Thermodynamics 53 (2012) 3035


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The percentage relative increase in the concentration of the to-tal aromatic components (TA%) is given by the following equation

[10]:

TA% xEA=1x

ES x

FA

xFA 100; 5

wherexES, andxFAare the mole fraction of solvent in the extract phase

and sum of the mole fractions of the aromatic components in thefeed, respectively.

Tables 10 and 11 contain the list of desirable properties that arementioned earlier for experimental and predicted data,respectively.

It should be mentioned that, some of these properties decreasewithOFwhile the others increase. Moreover, the properties whichdecrease with increase inOF, also show some differences in theirrate of change. Therefore, it is more appropriate to optimize the

product of these properties, which is named as processing solventindex (PSI), and define as follows[10]:

PSI% SPC%TA: 6

ThePSIvalues are obtained from experimental data (PSIexp.) as

well as predicted values by UNIFAC method (PSIpred.) and then theyare plotted versus the operating factor in figure 3. A polynomialbehaviour is exhibited by both. The experimental values (PSIexp.)and predicted values (PSIpred.) are fitted to the following equationsrelating the PSI to the operating factor:

PSIexp: 29OF3 96:34OF

2 10:65OF 39:2 103; 7

PSIpred: 19:96OF3 66:31OF

2 73:34OF 26:9 103:

8

Maximum values of (PSIexp.) and (PSIpred.) have been obtained bydifferentiation of the polynomial equations and are obtained asfollow:

OFexp:optimum 1:5011; 9

TABLE 7

Predicted (liquid + liquid) equilibria of the petroleum naphtha reformate with 40% CHP in solvent.

Component T= 300 K T= 310 K T= 320 K T= 330K



TABLE 9

Predicted (liquid + liquid) equilibria of the petroleum naphtha reformate with 45% CHP in solvent.

Component T= 300 K T= 310 K T= 320 K T= 330K



TABLE 8


Component T= 300 K T= 310 K T= 320K T= 330K


n-Hexane 0.037 0.005 0.039 0.007 0.037 0.007 0.035 0.01

Cyclohexane 0.074 0.01 0.081 0.012 0.079 0.016 0.078 0.021Benzene 0.027 0.022 0.03 0.024 0.027 0.026 0.026 0.022n-Heptane 0.085 0.013 0.085 0.014 0.081 0.015 0.079 0.014Toluene 0.265 0.129 0.281 0.128 0.27 0.139 0.273 0.127Iso-octane 0.045 0.007 0.049 0.005 0.061 0.006 0.064 0.004Ethylbenzene 0.036 0.017 0.035 0.014 0.035 0.018 0.033 0.016Xylene 0.138 0.06 0.136 0.054 0.133 0.054 0.135 0.051EC 0.136 0.565 0.123 0.562 0.15 0.554 0.148 0.561CHP 0.157 0.172 0.141 0.18 0.127 0.165 0.129 0.174



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OFpred:optimum 1:4648: 10

Solving Eq. (12) for different values ofTgives the corresponding

optimum values of (S/F) at which the extraction of this naphthareformate must be carried out.

OFoptimum Tm

T S

F PSI;Max

Constant; 11

S

F

optimum

constant T

Tm; 12

where the constant value for experimental data and predicted dataare 1.5011 according to Eq. (9) and 1.4648 according to Eq. (10),respectively.

These results are plotted infigure 4which is a plot of the opti-mized (S/F) ratio versus temperature (T). This figure indicates thatfor the extraction temperature in range of (300 to 330) K, byincreasing in temperature, it requires that extraction is carriedout at high (S/F) ratio in order to obtain the maximum value ofPSI. As can be observed infigure 4, the predicted and experimental

results show the same trend.

5. Conclusions

Experimental data show that a solvent with 40% CHP leads tothe most efficient performance for extraction of aromatic hydro-carbons from the investigated naphtha reformate within the tem-perature range of (300 to 330) K.

The UNIFAC model[6]was successfully used to predict phaseequilibria of extraction with a mixed solvent of CHP and EC. Theaverage RMSD between experimental and predicted values was4.86%

It is also concluded that the UNIFAC model [6]can successfullypredict the solvent capacity, selectivity and power and processingsolvent index (PSI) for aromatic extraction from the naphtha refor-mate using this solvent. Both temperature and (S/F) ratios affectthe (PSI). Atthe maximum(PSI), the corresponding operating factor(Tm/T)(S/F) was found to be 1.501 for experimental data and 1.464for predicted data. Using these optimum operating factor values, atrend has been obtained to determine the extraction temperaturefor a given solvent to feed ratio and vice versa. This can be usedindustrially to work at optimum conditions to extract aromaticsfrom naphtha reformate by ethylene carbonate and 1-cyclohexyl-2-pyrrolidone mixed solvent.

Acknowledgments

This work was financially supported by the Imam Khomeini OilRefining Company of Shazand/Iran.

FIGURE 4. Optimized operating conditions ((S/F)vs.T).

FIGURE 2. Aromatic content in extract phase for different temperatures anddifferent co-solvent compositions.

FIGURE 3. Effect of operating factor (OF) on processing solvent index (PSI).

TABLE 10

Desirable properties for extraction based on experimental data.

Property T= 300 K T= 310 K T= 320 K T= 330K

C 0.5306 0.4950 0.5699 0.5560S 3.3206 3.1594 3.1205 2.7927P 0.291 0.283 0.317 0.312TA/% 31.180 29.182 28.294 26.115

TABLE 11

Desirable properties for extraction based on predicted data.

Property T= 300 K T= 310 K T= 320 K T= 330K

C 0.5276 0.4914 0.5641 0.5506S 3.3220 3.1651 3.1029 2.8527P 0.2490 0.2485 0.2784 0.2801TA/% 31.133 30.389 28.844 27.688

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Extraction of aromatics from petroleum naphtha reformate by solvent.pdf