Ind. Eng. Chem. Res. 1994,33, 336-340
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Naphtha Desulfurization by Adsorption Abu Bakr S. H.Salem Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
Removal of sulfur compounds from naphtha solutions has been investigated by using adsorption methods. Activated carbon, zeolite 5A, and zeolite 13X were used for this purpose. Zeolite 13X has shown higher capacity for sulfur at low concentration ranges. At higher ranges the capacity of activated carbon is 3 times greater than that of 13X zeolite a t 20 "C. Zeolite 5A was unfavorable for sulfur sorption from naphtha. A correlation is developed to fit the experimental data. Agreement is reasonably good. Table 1. Physical ProDerties of the Adsorbents Used
Introduction The presence of sulfur compounds in petroleum fractions is highly undesirable since they result in corrosion and environmental problems. These compounds are also responsible of reducing the performance of engines using such fuels. Hydrodesulfurization is the conventional method in which the sulfur compounds are effectively reduced. However, in these processes severe conditions are required such as high temperature of 600-800 O F and high pressure which may reach 3000 psig. In addition, there is a high consumption of hydrogen and expensive cobalt molybdenum catalysts are used. Ali (1987) reported a study on the removal of mercaptans, sulfides, and disulfides from a petroleum oil of 240360 "C boiling range using 5A and 13X molecular sieves. Davis (1991) mentioned that certain zeolites are successfully used for the selective adsorption of polar or polarizable molecules such as water and C02 and sulfur-containing molecules from some petroleum fractions. These adsorbents are hydrophilic and contain large void fractions. Such synthetic zeolites have a well-defined crystalline lattice of metal alumina silicates which contains uniform pore sizes. This framework has a unique property of selectively adsorbing molecules on the basis of their size, configuration, polarity, and other physical characteristics (Ruthven and Goddard, 1984). Information concerning the relevant adsorption equlibria is generally an essential requirement for the analysis and design of an adsorption separation process. Experimental investigation should be undertaken to justify using adsorption techniques in removing sulfur, nitrogen, and other impurities from petroleum fractions. This study addresses the adsorption of sulfur compounds from naphtha solutions at 20 "C using activated carbon, zeolite 5A, and zeolite 13X. The objective is to find an economically attractive alternative method for hydrodesulfurization of petroleum fractions and to select a suitable adsorbing material for this process. The study shows that zeolite 13X and activated carbon are suitable adsorbents for sulfur removal from naphtha. Experimental Section and Results Charcoal granular activated carbon and zeolites 5A and 13X were obtained from BDH Chemicals Ltd. Some physical properties of these materials are given in Table 1. Naphtha solutions composed of 50150 virgin and catalytic cracked origins were obtained from SAMAREC Riyadh Refinery. The ASTM D-86 boiling range of the mixture is given in Table 2. This mixture contained 502 ppm sulfur. The solutions were further diluted with OSSS-5885/94/2633-0336$04.50/0
type zeolite 5A zeolite 13X activated carbon
form extrudate bead granule
size 1/16 in. 4-8 mesh 4-12 mesh
bulk density (kg/ms) 700.0 609.2 600-900
nominal pore diam (A) 5.0
10.0 4.0-9.0
Table 2. ASTM D-86. Boiling Range of the Naphtha Mixture simulated distillation bp ("C) simulated distillation bD ( O C )
IBP 10% distillation 30% distillation 50% distillation
65.0 93.0 112.0 128.0
70% distillation 90% distillation FBP
146.0 167.0 194.0
naphtha containing 2 ppm sulfur to yield solutions with about 50 ppm, which is equivalent to about 36.0 mg/L. Table 3 shows the gas chromatographic analysis of the naphtha solutions based on the number of carbon atoms. Prior to use in the adsorption isotherms determinations, the activated carbon was washed several times using deionized water to remove fines and subsequently dried in an oven under vacuum of 0.68 in Hg and at a temperature of 150 "C for 24 h. It was then placed in a vacuum desiccator until it was to be weighed for the adsorption study. The molecular sieves particles were treated in a similar way to that used for activation of the charcoal but at a temperature of 220 "C for 48 h. Adsorption isotherm data were determined by adding different amounts of adsorbents (weighed to 0,0001 g) to 50 mL of the naphtha solutions in separate 100-mL Erlenmeyer flasks which were closed by tightened stoppers made of glass and Teflon to avoid vapor losses during stirring. The solutions were then placed in a shaker bath for 2 h, which is double that required to reach equilibrium as initially proved by preliminary tests. The temperature of the bath was controlled at 80 "C to enhance the masstransfer process. The flasks were then removed from the bath and left at ambient temperature (20 "C) for 48 h for settling and equilibration. The solutions were filtered twice with a Whatman No. 3 filter paper to ensure that all the solid particles were removed from the liquid. Filtrate samples were analyzed by using a Houston Atlas Model 825RD/856 total sulfur hydrogenator. This device consists essentially of two units: a hydrogenator unit (Model 856) and an analyzer (Model 825 RD). In the hydrogenator, the sample is pyrolyzed in the presence of H2 at a temperature of about 1300 "C. At such temperatures the sulfur compounds are converted into H2S. The flow of HzS-bearing gas is directed to the analyzer unit. The analyzer contains a sensing tape impregnated with lead acetate on which H2S will form a brown stain. The 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 337 Table 3. Analysis of t h e Solutions of Naphtha Used paraffins carbon no. 5 6 7 8 9
satd 0.05 1.52 15.10 15.47 15.44 10 9.37 11 1.68 mercaptan sulfur (ppm) disulfide sulfur (pprn) thiophene sulfur (pprn) paraffins
carbon no. satd 5 0.20 6 1.76 7 14.99 8 15.23 9 14.54 10 9.29 11 1.95 mercaptan sulfur (pprn) disulfide sulfur (pprn) thiophene sulfur (ppm)
(a) Naphtha Containing 502 ppm Sulfur naphthenes unsatd satd unsatd 0.00 0.00 0.00 0.00 1.32 0.00 0.00 7.00 0.00 0.00 9.20 0.01 0.00 4.99 0.00 0.00 3.89 0.00 0.00 0.46 0.06 297 total sulfur (pprn) 5 density (kg/L) 200 refractive index (b) Naphtha Containing 50 ppm Sulfur naphthenes unsatd satd unsatd 0.00 0.01 0.00 0.00 1.32 0.00 7.64 0.00 0.00 8.95 0.00 0.00 0.00 5.66 0.00 0.00 3.96 0.00 0.12 0.53 0.06 29 total sulfur (pprn)
