Adsorptive separation of dimethyl disulfide from liquefied petroleum gas by different zeolites and selectivity study via FT-IR
Graphical abstract
Adsorbent Ag2O/NaY which was prepared by equivalent-volume impregnation method was selected and used for separation of dimethyl disulfide from model liquefied petroleum gas. The separation ability of NaY and Ag2O/NaY for removing dimethyl disulfide from model liquefied petroleum gas with or without 1-hexene was compared. The adsorption strength of dimethyl disulfide on NaY and Ag2O/NaY as well as the competitive adsorption of dimethyl disulfide and olefin on NaY and Ag2O/NaY were investigated detailedly by FT-IR spectroscopy.
Introduction
In the refinery, further processing of crude oil will produce plenty of liquefied petroleum gas (LPG). LPG contains large amounts of C3 and C4 components which can be used as fuel and petrochemical feedstock. However, LPG also contains sulfur compounds such as H2S and mercaptans, which are not only being well-known as catalyst poisons, but also their byproducts upon combustion (i.e., SO2 and SO3) are environmental pollutants. For all these reasons, desulfurization needs to be carried out when the LPG is for sale or as the petrochemical feedstock.
For the removal of mercaptans in LPG, the most extensively used technology is Merox process [1], which was developed by Universal Oil Products Company (UOP). The Merox process converts mercaptans to disulfides in alkaline solution using air as the oxidant in the presence of Merox catalyst. However, the major problem for ultradeep desulfurization of LPG by Merox process is that, being oxidation products, the disulfides such as dimethyl disulfide (DMDS) are difficult to be separated at the lye extraction step of Merox process [2]. This often leads to excessive sulfur content in LPG. If this kind of LPG is used as the petrochemical feedstock for the production of methyl tert-butyl ether (MTBE) and alkylates, which are commonly added to gasoline to boost the octane number of gasoline [3], it will eventually lead to the high sulfur content in gasoline. This kind of gasoline cannot meet the increasingly stringent standards (sulfur content less than 10 ppmw, European V Emissive Standard). Thus, the deep desulfurization of DMDS in LPG is important [4].
Up to present, the sulfide removal methods for hydrocarbon fuels mainly have the following several kinds, such as hydrodesulfurization (HDS) [5], [6], [7], selective catalytic oxidation (SCO) followed by adsorption [8] and adsorptive desulfurization [9], [10], [11], [12], [13], [14], [15]. HDS is a typical and very effective desulfurization process. However, HDS is operated at high temperature and pressure and requires addition of hydrogen [16]. Furthermore, during the desulfurization process, olefin may be lost significantly because of the conversion of olefin to alkane. Lampert [8] reported the desulfurization of natural gas and LPG by the SCO process. The sulfur compounds are oxidized to SOX under air atmosphere firstly, and then the adsorption of converted SOX is carried out at high temperature. While SCO has the advantage that it does not consume hydrogen, it has the disadvantage that the unconverted oxygen and nitrogen from the C4 hydrocarbon should be removed in an additional separation unit.
Comparing with the above methods, the adsorptive desulfurization has the advantages that it does not only can be operated at relative low temperature and pressure, but also requires no hydrogen. Thus, the adsorptive desulfurization is regarded as one of the most appropriate and promising methods.
In the previous studies, the most extensively studied adsorbent materials were activated carbon and zeolite. Cui and Turn [17] modified activated carbon (AC) with FeCl3 solution and reported that it has an improved adsorption capacity for dimethlysulfide (DMS) compared to the unmodified activated carbon. They inferred that the new active sites formed by the modification of FeCl3 on the carbon surface likely had stronger affinity with DMS molecule, leading to the improvement of the DMS adsorption capacity. Xiao et al. [18] studied the adsorption of dibenzothiophene (DBT) on Ag/Cu/Fe-supported ACs prepared by impregnation method. The results indicated that adsorption capacity followed the order: Ag/AC > Cu/AC > AC > Fe/AC. Activated carbon impregnated with CuCl and PdCl2 were used to remove methyl mercaptan (MM), DMDS and diethyl disulfide (DEDS) in Fluid Catalytic Cracking (FCC) C4 hydrocarbon mixture by Kim et al. [19]. The breakthrough experiments showed that the sulfur adsorption capacities of the adsorbents decreased in the following order: AC impregnated PdCl2, AC impregnated CuCl and non-impregnated AC.
Comparing with activated carbon, zeolites are more widely investigated as the adsorbents for its excellent thermal stability. Wakita et al. [20] have investigated the removal of DMS and t-butylmercaptan (TBM) in the city gas using Na-Y, Na-X, Ca-X, H-Y, USY, H-β and H-ZSM-5 zeolite. They concluded that the adsorption site of Na-Y was the Na+ in the supercage. Adsorptive removal of DMS and TBM from pipeline natural gas was also efficiently carried out by Satokawa et al. [21] using silver exchanged Y zeolite in the presence of water. They reported that the sulfur removal capacity can be improved by an increase of the silver content. Lee et al. [22] studied the adsorptive removal of tetrahydrothiophene (THT) and TBM using AgNa-Y zeolites. The Ag–Na-Y were also obtained via Ag+-exchange, so as to form active adsorption sites (Ag+, Na+, Ag0, H+, and Ag2O). According to the study, they found that the strongest adsorption site was Ag+ and the adsorption strength of THT on these adsorption sites was higher than that of TBM. Song et al. [23] used transition metal nitrate impregnated β-zeolite (BEA) to remove THT and TBM from city gas. They found that AgNO3 impregnated BEA showed the highest adsorption capacity among the metal nitrates used.
Gong et al. [24] have investigated the removal of thiophene and DBT in FCC gasoline using Cu(I) and Ag(I) metal exchanged β-zeolite. They reported that both adsorbents had similar sulfur removal capacity, when applied to the model gasoline. However, for the real FCC gasoline, the sulfur removal capacity was reduced greatly. They claimed that the reduction of sulfur removal capacity in real FCC gasoline was caused by the competitive adsorption between sulfur compounds and the olefin or aromatic component present in the real FCC gasoline.
LPG can be regarded as the mixed C3–C4 stream, which contains sulfur compounds and olefin compounds. The major sulfur compound found in LPG is DMDS, which accounts over 80% of total sulfur compounds in the LPG stream. Takatsu et al. [25] investigated the removal of DMDS in LPG by use of activated carbon, ZnO/Al2O3, Ag-exchanged β-zeolite and Ag/CeO2. Among the tested adsorbents, Ag-exchanged β-zeolite showed better adsorption capacity for sulfur compound DMDS. They deduced that the electrostatic attraction between Ag(I) and the sulfur atom of DMDS played a critical role in the adsorption. Nonetheless, the model gas they used contains 98% propane, and the impact of the olefin in LPG was not concerned. In fact, in most cases, the LPG stream is rich in olefin compounds. It is well known that the competitive adsorption between organic sulfur compounds and olefins is one of the crucial factors affecting adsorption selectivity for sulfur containing molecules from real fuels. However, few studies about the impact of olefins on the desulfurization of hydrocarbon fuels are found in the open literatures. Thus, it is necessary to consider the impact of olefin on the adsorption removal of DMDS in LPG.
For the adsorptive desulfurization of disulfide in olefin rich hydrocarbon fuels, Lee et al. [3] investigated the removal of DMDS in the olefin rich FCC C4 hydrocarbon mixture. They used the TPD method to investigate the adsorption strength of DMDS and 1-butene on Ag(I) exchanged β-zeolite. In addition, the fourier transform infrared (FT-IR) spectroscopy was also applied in the competitive adsorption mechanism study by Tian et al. [11] to investigate the competitive adsorption of thiophene and aromatics, and the results show that it is also an effective means.
In this paper, the removal of DMDS from LPG was investigated by using metal oxide loaded zeolite. The metal elements used were Ag, Cu, Ni, Ce, and Zn. The breakthrough experiments with model solution (1468 ppmw DMDS, i.e. 1000 ppmw S, 50 wt.% petroleum ether and 50 wt.% 1-hexene) were performed to select the best active component of adsorbent for the removal of DMDS. Content of active component and regeneration ability of adsorbent were also investigated. Furthermore, Fourier transform infrared (FT-IR) spectroscopy was employed to study the adsorption behavior of DMDS and olefin on adsorbents.
Section snippets
Material
Analytical reagent grade DMDS (Development of Beijing Chemical Technology Co., Ltd., 99.5%) was selected as the target sulfide. The olefin rich model LPG was prepared with 1000 ppmw of S in 50 wt.% petroleum ether (Sinopharm Chemical Reagent Co., Ltd., boiling range: 30–60 °C) and 50 wt.% 1-hexene (Aladdin Chemistry Co., Ltd., analytical reagent). While the model LPG without olefin was prepared by dissolving a certain amount of DMDS in organic solution petroleum ether to make sulfur content of
XRD and N2 adsorption–desorption characterizations
In comparison with NaY, the same diffraction peaks displayed in modified NaY indicate that the zeolite structure was maintained after impregnation and calcination (see Fig. 1). The data in Table 1 show the texture properties of NaY and modified NaY. It can be seen that the surface area and pore volume decreased when NaY was loaded with metal oxide active component.
Adsorption of DMDS onto 13X and NaY
A model solution with high sulfur concentration (1000 ppmw) was used to screen different adsorbents. The sulfur removal of DMDS from
Conclusion
In summary, the removal of DMDS from the model solution by adsorption on different adsorbents was conducted in a fixed-bed flow reactor, and the operational conditions on the sulfur removal performance of the adsorbent, as well as the adsorptive selectivity and the regenerability of the adsorbent were also investigated. The following conclusions were elicited by experiments:
- (1)
The breakthrough experiments showed that NaY exhibited better desulfurization performance than 13X. When loaded with
Acknowledgement
The authors are grateful for the financial support of the National Natural Science Foundation of China (Projects 21206008).
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