The success of any new industry lies in its ability to
innovate and grow. Future growth in renewable fuels may require
an evolution from first-generation products, such as ethanol and biodiesel, to
next-generation products, such as isobutanol.
Isobutanol, a form of biobutanol, has many outstanding
characteristics that allow it to be used in a variety of
- As isi.e., as a solvent or as a
- Converted, through known processes, to a variety of
hydrocarbons for use in the petrochemical and/or refining industries
- In existing production, distribution, marketing and
This article highlights the technology, feedstocks and market growth
opportunities for isobutanol, with a focus on potential new
market offerings in 2012.
Technology pathway for bio-isobutanol
The specialized production process for bio-isobutanol is
fermentation paired with an integrated separation technology. This approach, developed
over the past seven years, has been successfully proven at
bench scale, at a pilot plant, and at a 1
million-gallon-per-year (MMgpy) demonstration plant. In May
2012, the worlds first commercial, bio-based isobutanol
production plant was started up in Luverne, Minnesota, with a
capacity of 18 MMgpy.
Bio-isobutanol fermentation is very similar to the existing
ethanol process. Ethanol plants can be repurposed to make
isobutanol relatively easily and cost-effectively, with two key
- Modified biocatalyst. Isobutanol is a
naturally occurring product of the fermentation process,
found in many items such as bread and scotch whiskey;
however, its commercial use to date has been limited.
However, through innovations in microbiology and
biochemistry, traditional yeasts have been modified, making
possible a much higher selectivity in producing
isobutanoli.e., turning up the yeasts ability to
make isobutanol while also limiting the ethanol production
- Unique proprietary separation. As the
isobutanol is produced, a stream is taken from the
fermentation broth where the isobutanol is removed, and the
remaining broth is returned for further conversion. This has
the effect of keeping the isobutanol concentration below the
biocatalyst toxicity level, but it allows for improved
With these two additions to existing facilities, it is clear how the project completion time and CAPEX to
make bio-isobutanol can be significantly lower than those for
the construction of a greenfield plant.
A plant conversion can nominally be 20%40% of the CAPEX
of a greenfield bio-isobutanol plant. As fermentation ethanol
plants have been shut down or under-utilized due to recent poor
economics (e.g., the US ethanol subsidy has been repealed, and
the regulation blend wall has effectively been
reached), the ability to repurpose these plants to isobutanol
becomes an attractive opportunity.
Upon fermentation plant conversion, the plant capacity will
be approximately 80% on a volumetric product-yield basis
(compared to ethanol), but comparable on an energy-equivalent
basis (isobutanol contains more energy than ethanol).
Therefore, the utility requirements and OPEX are comparable to
ethanol production (which, again, limits CAPEX
There is over 20 billion gpy (Bgpy) of existing fermentation
ethanol capacity in the world, located mostly in North and
South America. A leading company in bio-isobutanol is
converting some of these ethanol plants to isobutanol
production. That companys business model is based on the
flexibility to buy ethanol plant assets, form a joint venture
with the current plant owner for the conversion, or to license
the isobutanol production technology to ethanol plant
Fig. 1 illustrates an isobutanol plant
conversion. The before photo shows a facility in
Luverne, Minnesota as a 22-MMgpy ethanol plant. The
after photo depicts the plant as it was repurposed
to produce up to 18 MMgpy of isobutanol.
1. Conversion of a fermentation ethanol
plant to an isobutanol plant in Luverne,
One companys proprietary fermentation process is
designed to convert feedstocks of all types: grain,
sugarcane, cellulose and/or nonfood-based materials. Almost
anything that can be converted into a fermentable sugar can be
used, whether it is a traditional C6 sugar, such as
glucose, or a C5 sugar, such as pentose. The issue
of feedstock selection is one of economics, but technology can
be put into yeasts to allow them to digest C6 or
C5 sugars. In fact, at bench scale, these yeasts
have produced cellulosic isobutanol using a mixed stream of
C5 and C6 sugars.
Bio-isobutanol has versatility
One of the main reasons that converted plants have such good
projected economics is that bio-isobutanol is versatile as a
platform molecule. In the chemicals arena, it can be sold as a
solvent product (e.g., paints) and/or converted into materials
such as butyl rubber, paraxylene (PX) and other derivatives for
use in market applications such as tires, plastic bottles,
carpets and clothing. (This conversion is accomplished through
dehydration to isobutylene.) For fuels applications, isobutanol
can be blended as a low-vapor-pressure gasoline component
and/or used as feedstock to make other transportation fuels
(e.g., iso-paraffinic kerosine for use as biojet) or other
renewable products (e.g., renewable heating oil).
Bio-isobutanol as a gasoline blendstock
Bio-isobutanols properties as a gasoline blendstock
can best be understood by comparing some of the blending
properties to ethanol and alkylate. Table 1
summarizes some key aspects in the comparison.
Compared to ethanol, isobutanol has a much lower Reid vapor
pressure (RVP) and about a 30% higher energy content. The blend
octane of isobutanol is high as well (although slightly lower
than ethanol). Isobutanol also has a lower oxygen
(O2) content than ethanol, so more isobutanol can be
blended into gasoline for a given O2 content.
Greater blend volume, plus higher energy content, means more
renewable identification number (RIN) generation. See
Table 2 for a RIN comparison summary.
Unlike ethanol, which is fully miscible in water, isobutanol
has limited water solubility (about 8.5%). Isobutanol also does
not cause stress corrosion cracking in pipelines. These factors
result in major advantages in terms of blending logistics.
Isobutanol can be blended as a drop-in renewable fuel at the refinery and shipped in pipelines to
fuel terminals via existing infrastructure, which prospectively
eliminates the need for segregated tankage or pipelines. This
also affords refiners the opportunity to once again produce a
finished-specification gasoline vs. a sub-octane blendstock for
Isobutanol overcomes the regulation blend wall
limitation of ethanol blending. Isobutanol blended into
gasoline up to 12.5 vol% produces a substantially similar
gasoline at a 2.7% O2 content. For refiners, this is
a conservative first step for blending, and it generates 16.25
RINs per gallon of finished product. E10 has 3.5 vol%
O2, which is the currently accepted limit of
O2 content by automobile engine manufacturers. For
this same 3.5 vol% O2, a US Environmental Protection Agency
(EPA) waiver (211b) exists that would potentially allow
isobutanol blending of up to 16.1 vol%, yielding 20.93 RINs, or
more than twice the number of RINs as E10 for an equivalent
Bio-isobutanol can be an advanced biofuel
To account for the relative amounts of renewable energy
benefit, each biofuel generates a RIN based on its energy
content. There are basically four types of RINs: renewable
(e.g., first-generation, corn-based ethanol), biomass-based
diesel, cellulosic and advanced.
Advanced RINs are generated with the production of advanced
biofuels with an approved US EPA pathway (i.e., rated as having
at least a 50% reduction in greenhouse gas footprint vs.
baseline hydrocarbon fuel). Since bio-isobutanol
has a higher energy content than ethanol, bio-isobutanol
generates 1.3 RINs per gallon, vs. first-generation
ethanols 1.0 RINs per gallon. In addition, whereas
todays corn ethanol is precluded from qualifying as an
advanced biofuel, bio-isobutanolproduced with a green
energy source (e.g., biomass-fired combined heat and power) has
the potential to qualify for advanced RIN status.
Fig. 2 summarizes the US Renewable Fuel
Standard (RFS) projected gallons for implemented renewable and
advanced biofuels, as compared to the requirements stated by
the US Energy Independence and Security Act (EISA) of 2007. As
can be seen, there is a projected shortfall of advanced
biofuels. Bio-isobutanol offers some flexibility for meeting
the RFS2 targets with domestically produced renewable fuels, as
opposed to relying on sugarcane ethanol imports from Brazil, which
is the main biofuel pathway currently approved by the EPA for
2. Projected RIN-gallons vs. EISA targets.
Bio-isobutanol as renewable feedstock for biojet
Taking the bio-isobutanol and processing it further to
isoparaffinic kerosine (IPK) biojet has been demonstrated at a
hydrocarbon plant in Silsbee, Texas. The process is outlined in
3. Isobutanol-to-IPK jet fuel
Producing IPK biojet from bio-isobutanol involves three
- Dehydration of the renewable isobutanol to
- Oligomerization of the isobutylene to mostly
trimers/tetramers to produce C12 and
- Hydrogenation of olefins to IPK biojet.
These processes present opportunities for retrofits of
existing, underutilized refining/petrochemical assets, in some cases.
Commercialization and integration into an existing process
plant should be straightforward.
Depending upon economics, the overall process also has the
flexibility to make more or less isooctene and/or isooctane
product streams, which make good renewable gasoline blending
components. It should be noted that both renewable gasoline
blendstocks (isobutanol and isooctene) are not tied to crude
oil processing, so these are not likely to have crude oil
volatility effects. Again, isobutylene, isooctene and isooctane
can also be drawn off for the production of other renewable
petrochemical products (e.g., PX).
This biojet process has been demonstrated in a small
(10,000-gallon-per-month-capacity) unit for several months. The
alcohol-to-jet (ATJ) product has been sold to the US Air Force
as part of the Alternative Fuels Certification Office (AFCO)
process. Fig. 4 shows a picture of the
demonstration plant in Silsbee, Texas.
4. IPK biojet demonstration plant.
IPK process steps
There are three steps in the IPK production process.
Dehydration. Step 1 is the dehydration of
isobutanol to isobutylene and water. The reaction is
endothermic, with a relatively low operating pressure (< 200
psig) and temperatures of around 550°F650°F. The
operating requirements are similar to semi-regenerative
catalytic reformingolder technology that has since been
upgraded in refineries and petrochemical plants. Therefore,
idled semi-regenerative reformers are possibilities for
retrofits to develop the dehydration step. The catalyst for the
dehydration has been fully commercialized in similar
The dehydration reaction can be efficiently designed to
almost complete conversion, minimizing the downstream
complexities of the separation of the butylene and water, and
the effluence of the water.
It should be noted that isobutylene can be a hydrocarbon
feedstock for other refining and petrochemical processes. Since
the isobutylene is renewable, any resulting RINs would carry
forward to any hydrocarbon product covered by RFS2.
Oligomerization. Step 2 is the
oligomerization of the isobutylene to dimers (isooctene),
trimers (C12 olefins) and tetramers. There is some
measure of flexibility in the amount of each olefin produced.
Since IPK jet fuel primarily requires
C12C16 olefins, dimers are recycled
to yield more trimer/tetramer product.
Oligomerization is an exothermic reaction, with operating
conditions, heats of reaction, and catalysts that closely
resemble MTBE production units and/or catalytic polymerization
units; these units are possible retrofit candidates for this
oligomerization step. In fact, after MTBE was banned in the US,
many MTBE units were converted to make isooctene (dimer). These
units could be used with a minor retrofit. Depending upon
economics, the dimer could be used for gasoline blending and/or
further processing options.
Hydrogenation. Step 3 is the saturation of
the olefin product from the oligomerization section. This is
also a well-known and practiced operation in refineries and
petrochemical plants. The main reaction is the conversion of
the trimers/tetramers to IPK. The operating conditions are
mild, and they have relatively low operating pressure and
temperature, and modest space velocity requirements. The
hydrogenation reaction is exothermic and occurs with hydrogen
consumption in the process, so some recycle and cooling design
details are correlated with the reactor bed design to ensure
proper heat removal and control of the reaction.
Olefin hydrogenation is well-known and practiced, so there
may be an opportunity to retrofit existing assets, since
lower-pressure hydrogenation units have been idled as
hydrogenation requirements have become more severe. The
operations learning curve is somewhat established already, as
per catalyst preparation, unit startup, normal plant
IPK biojet has some properties that enhance its value. The
freeze point is low (80°C), while oxidation stability
is high. Starting from isobutanol, a renewable IPK would also
generate RINs at the rate of 1.6 per gallon, based on the
process. The current specification limit for a jet fuel blend
with synthetic blending components is a maximum of 50%. For a
1:1 blend with petroleum jet fuel, 80 RINs are generated for
every 50 gallons of IPK that are used to produce 100 gallons of
blended jet product.
Scoping economics of biojet
One important aspect of understanding how bio-isobutanol can
be a versatile alternative biofuel is the nominal economic
incentive for its conversion to jet fuel. Preliminary scoping
economics were developed for making biojet from renewable
isobutanol feedstock. Although a retrofit of existing units
would help the economics, retrofits are not possible in all
cases. Therefore, a new unit was used as the basis for this
In addition to CAPEX and efficiencies associated with the
possible retrofit of some existing assets, the other
sensitivity in scoping economics is the value and use of
established RIN and other tax credit incentives, as
The CAPEX throughput basis was a nominal
3,000-barrels-per-stream-day (bpsd) grassroots plant. The unit
was assumed with all new equipment (no retrofit or surplus or
idled equipment). All inside-battery-limit (ISBL) equipment was
sized, specified and budget-estimated. The CAPEX was determined
by applying factors to the equipment pricing to account for
commodity materials and labor. Allowances were also made for
engineering, escalation and contingency. A 30% allowance for
offsites was assumed and added.
For the jet fuel price basis, a relatively conservative
$2.60$3.40-per-gallon price range was assumed, although
the price could be higher. Sensitivities for this price range
were included in the scoping economic study.
With the advent of the jet fuel carbon tax on international
flights landing in the EU, the airline industry and fuel
suppliers have been looking for cost-effective, renewable
alternatives to petroleum jet fuel. A scoping sensitivity
examining this tax credit is shown in Fig. 5.
5. Biojet plant financial summary
Source: Mustang Engineering.
As can be seen, the EU tax credit has a significant effect
on the scoping economics. As one might expect, the RIN value
also has a considerable impact. In summary, this nominal
3,000-bpsd biojet plant study illustrated some positive scoping
economics, even at conservative jet fuel prices.
Bio-isobutanol for renewable PX for PET
Once the renewable hydrocarbon is made, there is the chance
to make renewable hydrocarbon products via traditional or even
newer processes. One new process uses isooctene to make PX,
which then can be made into purified terephthalic acid (PTA),
and then into renewable polyethylene terephthalate (PET) via
A pilot plant is being designed for this new process, which
yields PX at a very high selectivity vs. other xylenes. High
selectivity eliminates the need for xylene isomerization,
separation and recycle steps. Additionally, the PX can be
integrated with the rest of the biofuel plant, as shown in
Fig. 6. Depending on the relative amounts of
each renewable product, even the hydrogen made in the PX plant
can be used in the biojet hydrogenation unit.
6. Bio-isobutanol to paraxylene,
blendstock and/or biojet.
Isobutanol has gasoline blending, chemical and usage
advantages vs. ethanol, which result in positive economics for
the conversion of existing ethanol facilities to bio-isobutanol
production. Compared to other transportation fuel blendstocks,
bio-isobutanol is a better environmental alternative (e.g., low
vapor pressure, meaning lower volatility in finished fuel).
Also, being made by fermentation of sugars (via normal or
cellulosic biomass), these renewable fuels are not tied to
crude oil prices or to petroleum supply fluctuations.
The process configuration for bio-isobutanol to IPK biojet
fuel involves three sequential, straightforward steps. The
process operates at moderate operating conditions, and it is
similar to some existing refinery and petrochemical units that have been
idled or underutilized. Revamps are possible, and they would
reduce the CAPEX and construction time. Projected RIN
values and EU carbon tax incentives would provide additional
upside on the project economics. This three-step process has
been demonstrated at a 10,000-gallon-per-month-capacity hydrocarbon plant in Silsbee, Texas.
On-spec product is being made and sold to the US Air Force for
the military certification process.
Bio-isobutanol has numerous process and product platforms
that can be employed as economics dictate. These include, but
are not limited to, solvent sales, use as a gasoline
blendstock, conversion to biojet or use as a feedstock for
renewable PX. Bio-isobutanol has the versatility to allow
multiple options at the same time. For example, marine and
small-engine fuels are niche options that can be addressed.
Renewable diesel is another option.
The pathway for bio-isobutanol via fermentation has been
established, and the business model makes economic sense to
revamp idled or underutilized fermentation ethanol plants. One
companys production of bio-isobutanol at demonstration
scale was proven in 2009. More recently, a commercial-scale,
18-MMgpy plant was started up.
Furthermore, bio-isobutanol has versatility and environmental and economic
advantages when compared to ethanol. Bio-isobutanol has the
capability to provide significant impact as an advanced
gasoline blendstock, or as a feedstock to make other advanced
fuels or products; therefore, it should be considered a
high-potential, next-generation biofuel.
Rick Kolodziej is a process
technology manager at Wood Group Mustang. He has over
30 years of experience in process and project
engineering and development in the refining, petrochemicals, chemicals,
polymers and gas processing industries. Mr. Kolodziej
has been involved with several new technology
development projects, including several bio-related
projects. Most recently, Mr. Kolodziej was involved
with Gevos projects in renewable isobutanol and
various petrochemicals. He is also
responsible for process plant project development for
Wood Group Mustang in the Far East. Mr. Kolodziej has
US and international patents in hydrotreatment technology. He holds a BS
degree in chemical engineering from the University of
Illinois (Chicago) and an MBA degree in finance from
DePaul University, and is a registered professional
engineer in the state of Illinois.
Jeff Scheib is vice president for
fuels at Gevo Inc., overseeing sales, marketing and
business development activities for
isobutanol-into-fuels markets, including refining, biojet, gasoline
distributors and marketers, marine and small-engine
applications. He has over 20 years of fuels and biofuels leadership
expertise, having worked 17 years within the
petroleum sector with ARCO and BP, followed by four
years in the renewable energy arena with Cilion and
Chromatin, prior to joining Gevo in 2011. Jeff holds
an MBA degree from the University of California (Los
Angeles) and a BS degree in industrial engineering
from Northwestern University.