(88e) From Biomass to Ethylene: Steam Cracking of Bio-Synfined Naphtha

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 high oil demand from new economies, on the other hand the
growing awareness of the declining oil reserves. Many scientists claim that
global production of oil is set to peak in the next decade before entering a
steepening decline.  According to this "peak oil" theory¹ 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 the
fact that these oil fractions contain less naphtha, i.e. the main feedstock for
ethylene and propylene production, because of their heavier nature. Hence, even
if we ignore the fact that these reserves are more difficult to reach because
of their deep sea or Polar region location, 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. Carbon dioxide released during
the energy conversion of biomass (such as combustion, gasification, pyrolysis, anaerobic
digestion or fermentation) circulates through the biosphere, and is reabsorbed
in equivalent stores of biomass through photosynthesis.

A promising
route for the production of ethylene from wastes and renewable fractions is the
combination of the Bio-Synfining? process with a traditional steam cracker. Syntroleum's
patent-pending^3,4 Bio-Synfining? process catalytically
converts the triglycerides and/or fatty acids from fats, algae and vegetable
oils to a high quality synthetic paraffinic kerosene (SPK) or diesel and a
renewable naphtha in three steps. First, the raw feedstocks are treated to
remove catalyst poisons and water. In the second step, the fatty acid chains
are deoxygenated and transformed into mainly paraffins in a hydrotreater.  For
most bio-oils, fats, and greases, the hydrotreater liquid product consists
mainly of C15-C18 paraffins. In the third step of the process, these long
straight-chain paraffins are hydrocracked into shorter branched paraffins. The
hydrocracked products fall mainly in the kerosene and naphtha boiling range. 

The current contribution
will discuss on the one hand details about the Bio-Synfining? Process for
production of naphtha, including process technology, production capacity and
economics. Also recent progress on the Dynamic Fuels bio-refinery, scheduled
for startup in early 2010, will be reported. The latter uses Syntroleum's Bio-Synfining?
Technology to convert animal fats and greases provided by Tyson Foods to
naphtha and kerosene/diesel.

On the other
hand the contribution will discuss the results of an extensive steam cracking pilot
plant campaign performed with renewable naphtha from the Bio-Synfining? process.
This study was performed on the pilot plant of the Laboratorium voor Chemische
Technologie ^5,6 (LCT) in Ghent university and involved
the detailed characterization of the naphtha using GCxGC TOF-MS and GCxGC FID important
for addressing the quality of the produced naphtha (detailed PIONA, sulfur,
nitrogen and oxygenates content, etc.). Cracking of the
renewable naphtha leads to high light olefin yields and low amounts of
aromatics. For a residence time of 0.4 s, a Coil Outlet Pressure (COP) of 1.7
bar, a dilution of 0.45 kg steam per kg hydrocarbons and Coil Outlet Temperature
(COT) of 850°C, the renewable naphtha gives an ethylene yield of 31 wt% and a
propylene yield of 17.5 wt%. The absence of a significant naphthenic and
aromatic fraction in the feed results in low C5+ yields, low amounts of
pyrolysis gasoline and an  almost non-existing amount of pyrolysis fuel oil.
The influence of the process conditions on the product yields is as expected.⁷ At higher severities more
methane, ethylene, 1,3-cyclopentadiene, benzene and toluene are formed. The propylene
yield is highest at a COT of 820°C. Higher dilutions result in higher light
olefin yields and lower amounts of C5+ products for identical process-gas-temperature
profiles. Comparison with results obtained of several oil derived naphthas
shows that the renewable naphtha can be considered as a very attractive
feedstock for a steam cracker. The latter is confirmed by coking runs [COT = 850°C;
d = 0.45 kg/kg; COP = 1.7 bar] with and without DMDS addition and
comparison with coking data of reference feeds. The renewable naphtha has a low
coking tendency, and long run lengths can be expected. The
results obtained in this study are scaled-up to industrial furnaces (Lummus SRT-VI,
Millisecond, USC Shaw Stone&Webster) using COILSIM1D^8,9, resulting in product yields and run
lengths for a typical range of industrial operating conditions.   

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