Biomass to olefins: Cracking of renewable naphtha
Highlights
► Steam cracking of renewable naphtha was studied in a pilot plant. ► The naphtha was derived from waste fats and greases by catalytic hydrotreatment. ► The pilot plant experiments show that high light olefin yields can be obtained. ► Based on an experimental coking study, high run lengths can be expected.
Introduction
The recent high oil prices have focused research attention to alternative routes and feedstocks for the production of light olefins. The former was caused by two reasons: on the one hand, the increased oil demand from new economies and on the other hand, the growing awareness of the decline of oil reserves. Many scientists claim that the global oil production is set to peak in the next decade before entering a steepening decline. According to this “peak oil” theory [1] our consumption of oil will catch, then outstrip our discovery of new reserves and we will begin to deplete known reserves. Equally important for steam cracking is that these new oil fractions contain less naphtha, i.e. the main feedstock for ethylene and propylene production, because of their heavier nature. Hence, alternative routes for the production of light olefins should be considered. The use of renewable feedstocks is also encouraged by the growing awareness about the greenhouse effect [2], which is now generally recognized as being caused by the use of fossil fuels. The use of biomass for energy, fuels or chemicals is essentially carbon neutral. The quantity of carbon dioxide released during the conversion of biomass by combustion, gasification, pyrolysis, anaerobic digestion or fermentation corresponds to the amount fixed in it.
While most olefins are currently produced through steam cracking routes, they can also possibly be produced from natural or biogas via methanol [3], [4], [5] and oxidative coupling routes [6], [7]. Coal [8], [9], [10] and biomass-based routes [11], [12], [13], [14], [15], [16] have also been reported. The total energy use of the conventional routes is the lowest whereas that of the methane-based routes is 30% higher, and that of the coal and biomass-based routes is about 60–150% higher [17]. On the other hand production costs in the period 2030–2050 seem to be more favorable for coal and biomass based routes [17]. Other processes for producing bio-based olefins, such as fluidized catalytic cracking of vegetable oils, have been described in the patent literature [18]. The corresponding olefin yields are low compared to steam cracking of hydrocarbons. Furthermore, these alternative processes require design and construction of new facilities which are unlikely to match the economies of scale realized by existing ethylene plants. Use of bio-based hydrocarbons to supplement petroleum-based feeds to existing steam crackers thus seems the most viable near term approach to the transition of the commodity organic chemicals industry from petroleum towards renewable feeds, with little or no capital costs to olefin producers.
The Bio-Synfining™ process provides opportunities in this context to produce hydrocarbon feeds for conventional steam crackers. The patent-pending process [19], [20], [21] is represented schematically in Fig. 1. It catalytically converts the fatty acids and/or glycerides in waste fats/greases, vegetable oils, and algae, into paraffinic hydrocarbon fractions containing virtually no residual oxygenates.
The Bio-Synfining™ chemistry may be summarized using oleic acid as model component:C17H33COOH + 4H2 → n-C18H38 + 2H2On-C18H38 + H2 → n-C8H18 + i-C10H22
The first reaction, the so-called hydrodeoxygenation (HDO), is the main reaction during bio oil hydrotreating – the first of two hydroconversion steps in the process. The fatty acid chains are first hydrogenated and deoxygenated into straight chain paraffins. When the fatty acids are attached to a glycerol backbone (i.e. triglycerides), propane is produced in addition to n-paraffins. In the second step, the long straight chain paraffins are hydrocracked into shorter straight and branched paraffins (iso-paraffins). The hydrocracker product consists of a broad distribution of mainly C3–C18 hydrocarbons, the C4–C10 fraction being a renewable naphtha. The latter may be efficiently converted to the desired light olefins in a steam cracker. Therefore its conversion to olefins was studied in the steam cracking pilot plant set-up of the Laboratorium voor Chemische Technologie (LCT) of Ghent University. First a detailed characterization of the naphtha was carried out, followed by yield determination under varying process conditions. Finally a study on the coking tendency was performed with and without addition of dimethyl disulfide (DMDS). The results obtained in this study are scaled-up to industrial furnaces using COILSIM1D, relating the measured product yields and coking rates to expected run length for a typical naphtha furnace.
Section snippets
Feed pretreatment/hydrodeoxygenation/hydrocracking
Table 1 provides the make-up of the initial feed composed of low value waste fats and greases. Feed pretreatment is required when processing low value and waste bio oils. Typical contaminants include animal solids, phosphorus, and solubilized alkali/alkaline earth metals. The feed pretreatment process consisted of four steps:
- 1
Initial filtration of bio oil to remove the bulk of large particles and solids.
- 2
Washing with aqueous acid at a ratio of 10:1 (oil:water) to remove the solubilized metals and
Renewable naphtha production
Results from initial analysis of the bio oil blend show that the acid number is 94.7 mg/g KOH. This corresponds to a free fatty acid (FFA) content of about 47 wt%. The pretreatment is carried out according to procedure discussed in Section 2.1. Excellent contaminant removal efficiencies were obtained, acceptable for the hydrodeoxygenation step of the process.
The yield of paraffinic hydrocarbons from bio oil was 84.9% kilogram hydrocarbon per kilogram feed via HDO. This is near the theortical
Conclusions
It can be concluded that steam cracking of the naphtha obtained by the Bio-Synfining™ process provides high yields for the desired light olefins ethene and propene without even traces of oxygenates. The high yields are in line with the highly paraffinic nature of this naphtha. Also, its coking tendency is low compared to traditionally used fossil based naphtha fractions. Hence, a viable alternative for these fossil based feedstock is provided to existing steam cracking units.
Acknowledgments
KVG holds a Postdoctoral Fellowship of the Fund for Scientific Research Flanders and BOF tenure track position at Ghent University. The financial support from the Long Term Structural Methusalem Funding by the Flemish Government – grant number BOF09/01M00409 is acknowledged.
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