Polymeric carbon material from waste sulfuric acid of alkylation and its application in biodiesel production
Graphical abstract
Introduction
Alkylation of isobutane with butene using concentrated sulfuric acid is one of the important processes for the production of gasoline components with high octane number (Wang, H. et al., 2017). At the end of the reaction, the waste sulfuric acid is a kind of viscous liquid with black red color, generally referred as “waste sulfuric acid of alkylation” (WSAA). WSAA is a mixture containing about 90 wt % sulfuric acid and 6–7 wt % of acid soluble oil (ASO) and some water. The ASO consists of sulfuric acid esters and cyclic polyolefinic hydrocarbons having 2–4 double bonds, which show a wide range of boiling points (Berenblyum et al., 2002; Simon miron, 1962). In 2016, the capacity of China's sulfuric acid alkylation plants was more than 15 × 106 t, and the annual amount of WSAA was over 106 t, of which more than 60,000 t was ASO. It is costly for industry plants to regenerate the WSAA by high temperature incineration and the contained ASO is usually oxidized into CO2 and H2O. Regeneration of WSAA using electrochemical method has been reported (Albright, 2009), and another method is to treat the used acid propylene to form isopropyl sulfates, which was then extracted by isobutene (Chen, 1996). However, these methods can't be industrialized because of low efficiency and high cost. It's necessary to develop alternative technologies to treat WSAA with relatively low cost and obtain some value-added products.
On the other hand, the increasingly serious environmental problems caused by the burning of fossil fuels motivate the imminent development of biofuels (Ragauskas et al., 2006; Savage, 2011). Biodiesel is considered as a typical “green energy” and an alternative source because of its sustainable, biodegradable and non-toxic characteristics (Lai Fatt Chuah, 2017; Lin et al., 2011). In the green chemical industry, solid acid catalysts (SAC) are widely used because of their low corrosion, easy recovery and reusability. Synthesis of biodiesel using SAC as a catalyst can use both high acid value used oils and low grade non-refined oil as the feedstock (Wang, Y.-T. et al., 2017). At present, a series of SACs have been developed for the green synthesis of biodiesel (Bala et al., 2017; Lau et al., 2016) which include acidic oxide, heteropolyacid catalysts, sulfated zirconia based catalysts, organically-functionalized acid or sulfonated carbon materials (Martínez et al., 2018; Su and Guo, 2014). The sulfonated carbon materials (Liu et al., 2013) are a new kind of SAC, which shows good performance and attracts a lot of attention in recent years since they can be produced from renewable materials like sugar (Toda et al., 2005), rice husk (Zeng et al., 2016), cyclodextrin (Fu et al., 2015), biomass (Zhang et al., 2015) or other wastes (Bennett et al., 2016; Nurfitri et al., 2013). However, the preparation of the sulfonated carbon materials uses sulfuric acid as the sulfonate agent, where a large amount of waste sulfuric acid is produced. Therefore, a green, simple and cost efficient process for the production of sulfonated carbon materials is attractive.
In this study, a simple and cost effective process for the treatment of WSAA was proposed. This process was implemented by simply heating the WSAA at appropriate temperatures so that most of the ASO could be converted into polymeric carbon material (PCM), a kind of sulfonated carbon material which could be easily separated from sulfuric acid. The residual sulfuric acid can then be used to produce sulphates, such as ammonium sulfate, and the PCM can be used as SAC with relatively high acidity that could be applied to catalyze biodiesel production. This study provides a novel idea for the effective utilization of the waste materials in the energy production industry so that the amounts of waste chemicals could be significantly reduced.
Section snippets
Materials
Sulfuric acid (98.0 wt %), sodium chloride, methanol (99.5 wt %), oleic acid, phenolphthalein, potassium phthalate and sodium hydroxide were supplied by Beijing Chemical Works, Beijing, P.R.China. The WSAA was collected from Puyang Shengyuan Petroleum Chemical Industry Co., LTD (Henan, China). All the reagents used are as received, unless otherwise specified.
Determination of sulfuric acid and ASO content
The method to determine the amount of ASO in waste sulfuric acid was firstly proposed by Albright was used to calculate the concentration
Determination of ASO content by COD method
ASO of WSAA is the hydrocarbons mainly consisting of conjunct polymers and sulfate esters (Berenblyum et al., 2002). Therefore, it is practicable to determine the content of ASO by measuring the COD value of the WSAA. A certain amount of ASO was prepared according to procedure described in Experimental section of laboratory to determine the relationship between ASO and COD. The amounts of ASO produced at different treating time were different, and the results are shown in Table 1.
The
Conclusions
This process of separating ASO from sulfuric acid by polymerization is more energy efficient and less carbon emissions than pyrolysis process. At 453.15K, the removal of organic in WSAA was higher than 99.5 wt %. The PCMs are amorphous materials with -SH, S=O, -SO3H functional groups and high total acid densities, which could efficiently catalyze the oleic acid esterification with methanol to produce biodiesel. The conversion rate of oleic acid was 95.03 wt % with the optimized reaction
Acknowledgements
This work was supported by the National Key R&D Program of China (2017ZX07402003), the National Natural Science Foundation of China (U1610222) and the Chinese Academy of Sciences (CAS) Key Technology Talent Program of china. We also appreciate Prof. Wang Jidong of Beijing University of Chemical Technology for the help in this paper.
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