Biomass to olefins: Cracking of renewable naphtha

Abstract

An alternative route for the production of light olefins is proposed starting from low value and waste fats, greases and other renewable fractions. The first step catalytically converts triglycerides and/or fatty acids from bio oils to a high quality paraffinic diesel or jet fuel and renewable naphtha. GC × GC–TOF-MS and GC × GC–FID characterization of the renewable naphtha showed that it mainly consists of n-paraffins (32.6 wt%) and iso-paraffins (60 wt%), with only small amounts of aromatics (0.8 wt%), naphthenics (6.3 wt%) and olefinics (0.3 wt%). No remaining oxygenates are measured, making it a potentially attractive feed for the production of ethylene. Steam cracking of this renewable naphtha in a pilot plant revealed that high light olefin yields can be obtained (ethylene yield of 31 wt% and a propylene yield of 17.5 wt%), while the amount of pyrolysis gasoline (15 wt%) and pyrolysis fuel oil (<1 wt%) produced remains small. An experimental coking study further confirmed the attractive character of this feed. Run length simulations show that higher run lengths can be expected in comparison with the typically used naphtha fractions from fossil resources.

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 + 2H₂O

n-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.