Elsevier

Bioresource Technology

Volume 126, December 2012, Pages 48-55
Bioresource Technology

Wood-derived olefins by steam cracking of hydrodeoxygenated tall oils

https://doi.org/10.1016/j.biortech.2012.09.037Get rights and content

Abstract

Tall oil fractions obtained from Norwegian spruce pulping were hydrodeoxygenated (HDO) at pilot scale using a commercial NiMo hydrotreating catalyst. Comprehensive two dimensional gas chromatography (GC × GC) showed that HDO of both tall oil fatty acids (TOFA) and distilled tall oil (DTO) produced highly paraffinic hydrocarbon liquids. The hydrotreated fractions also contained fatty acid methyl esters and norabietane and norabietatriene isomers. Steam cracking of HDO–TOFA in a pilot plant revealed that high light olefin yields can be obtained, with 35.4 wt.% of ethene and 18.2 wt.% of propene at a coil outlet pressure (COP) of 1.7 bara, a dilution of 0.45 kgsteam/kgHDO–TOFA and a coil outlet temperature (COT) of 820 °C. A pilot plant coking experiment indicated that cracking of HDO–TOFA at a COT of 850 °C results in limited fouling in the reactor. Co-cracking of HDO tall oil fractions with a typical fossil-based naphtha showed improved selectivity to desired light olefins, further demonstrating the potential of large scale olefin production from hydrotreated tall oil fractions in conventional crackers.

Highlights

► Tall oil fatty acid and distilled tall oil hydrodeoxygenation produces paraffinic liquids. ► Steam cracking of hydrodeoxygenated tall oils at pilot plant scale. ► High light olefin yields when cracking hydrodeoxygenated tall oil fatty acids. ► Pilot plant cokes test indicates that reasonable run lengths can be expected.

Introduction

Ethene, propene, 1.3-butadiene, benzene, toluene and xylenes are produced and consumed in enormous amounts every year, as these base chemicals are the building blocks for most polymers and the starting materials for many additives, solvents, and other high-value chemicals. Currently, steam cracking of fossil feedstocks is mainly responsible for their production. For example 140 × 106 tons of ethene were produced in 2010 with an estimated growth rate of 5.3% (Zimmermann and Walzl, 2009). Therefore, producing these chemicals from renewable resources represents an enormous opportunity, and would contribute to the transition from a petrochemical to a green chemical industry.

Recent research has focused on several alternative technologies for production of olefins such as bio-ethanol dehydration (Kagyrmanova et al., 2011), methanol-to-olefins (MTO) (Chen et al., 2005), catalytic fast pyrolysis of lignocellulosic biomass (Carlson et al., 2011, Lavoie et al., 2011), bio-oil upgrading (Gong et al., 2011), etc. However, despite these research efforts and despite continuously increasing oil prices, petroleum conversion and petrochemical production processes are still highly profitable and crude oil remains the most important resource used by the chemical industry. The main reasons are the magnitude of past investments and the operating scale of current units. The use of biomass-derived feedstocks in existing conversion and production units is therefore a very interesting option, since it would allow production of renewable fuels and chemicals without the need to build new production facilities (Huber and Corma, 2007). For example, fluid catalytic cracking (FCC) of vegetable oils, or their mixtures with vacuum gas oil, using conventional FCC technology has already been studied (Bielansky et al., 2011, Dupain et al., 2007, Melero et al., 2010). However, although process conditions can be optimized to maximize propene yields, FCC is mainly used to produce liquid fuels. Alternatively, several types of low cost biomass resources are, after effective upgrading, suitable renewable feeds for conventional steam cracking (Kubičková and Kubička, 2010, Pyl et al., 2011a). For example, crude tall oil, a viscous liquid obtained as a by-product of the Kraft process for wood pulp manufacture (Norlin, 2005), can be fractionated into tall oil fractions called distilled tall oil (DTO) and tall oil fatty acids (TOFA). These fractions mainly contain long chain acids such as oleic and palmitic acids, and a certain amount of rosin acids, i.e. a mixture of organic acids such as abietic acid. Globally about 2 million tons of crude tall oil are refined annually in plants with a typical capacity of 100 000 t/a (Norlin, 2005). These tall oil fractions therefore meet the criteria of an economically desirable and readily available feedstock. However, removal of oxygen present in these fractions prior to steam cracking is crucial because separation sections of current steam cracking units are not designed to handle substantial amounts of oxygenated components. Additionally, these componets are considered to be the cause of safety issues related to fouling and gum formation in downstream processing (Kohler, 1991). Catalytic hydrodeoxygenation (HDO), using existing hydrotreatment technology and catalysts, of both DTO as well as TOFA removes the oxygen in the fatty and rosin acids in the form of H2O, CO and CO2, producing highly paraffinic liquids, i.e. HDO–TOFA and HDO–DTO, respectively (Anthonykutty et al., 2011, Harlin, 2012, Mäki-Arvela et al., 2011, Rozmysłowicz et al., 2010, Sunde et al., 2011). Such paraffinic liquids are commonly considered very desirable feedstocks for steam cracking due to the high light olefin yields they tend to attain (Kubičková and Kubička, 2010, Pyl et al., 2011a).

To properly assess the potential of such hydrodeoxygenated tall oil fractions, steam cracking of HDO–TOFA was studied in a gas-fired pilot plant equipped with a dedicated on-line analysis section. The latter includes a comprehensive 2D gas chromatograph (GC × GC) and enables quantitative and qualitative on-line analyses of the entire reactor effluent with high level of detail.

Furthermore, a pilot plant coking experiment was performed to qualitatively asses the coking tendency of the renewable feed at typical process conditions. Finally, because the enormous operating scale of industrial steam cracking requires vast amounts of feedstock, co-cracking of HDO–TOFA as well as HDO–DTO with petroleum-derived naphtha, the current the most common liquid steam cracker feedstock, was investigated.

Section snippets

Wood-derived feedstocks

The renewable liquids used as feedstock in the pilot plant steam cracking experiments were produced by hydrodeoxygenation (HDO) of commercially available tall oil fatty acid (TOFA) and distilled tall oil (DTO) fractions (Stora Enso Kraft pulping facilities, Sweden), obtained from Norwegian spruce pulping The raw TOFA had a high fatty acid content (92.6 wt.%) and a low content of rosin acids (1.3 wt.%) and unsaponifiables (6.1 wt.%). The DTO contained 23 wt.% rosin acids, 71.3 wt.% fatty acids and 5.7

Feedstock analyses

In Table 2, group-type composition and distillation data of the hydrodeoxygenated tall oils are presented as well as those of a typical petroleum-derived naphtha and natural gas condensate, i.e. common liquid feedstocks for current steam crackers (Zimmermann and Walzl, 2009).

Overall, approximately 100 components were measured in these mixtures, including paraffins, naphthenes, aromatics and methyl-esters. Fig. 1(a) shows the GC × GC–FID chromatogram of HDO–TOFA, indicating that it mostly

Conclusions

Catalytic hydrodeoxygenation of tall oil fractions produces paraffinic liquids. These liquids are attractive renewable feedstocks that can be used in conventional steam cracking units for the production of green olefins. Compared to typical petroleum- or natural gas-derived feedstocks, high amounts of ethene and propene can be obtained. Moreover, reasonably high run-lengths can be expected. Co-cracking of HDO–TOFA with naphtha will require a compromise between optimal process conditions,

Acknowledgements

The authors acknowledge Stora Enso for supporting this research, and in particular Jari Räsänen and Tapani Penttinen for their vision on wood based olefins. The authors also acknowledge the financial support from the Long Term Structural Methusalem Funding by the Flemish Government – grant number BOF09/01M00409.

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