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 ways:
- As isi.e., as a solvent or as a gasoline blendstock
- Converted, through known processes, to a variety of hydrocarbons for use in the petrochemical and/or refining industries
- In existing production, distribution, marketing and end-user assets.
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 modifications:
- 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 pathway.
- 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 conversion.
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 requirements).
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 owners.
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.
| Fig. 1. Conversion of a fermentation ethanol |
plant to an isobutanol plant in Luverne, Minnesota.
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 oxygenate blending.
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 O2 content.
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 advanced status.
| Fig. 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 Fig. 3.
| Fig. 3. Isobutanol-to-IPK jet fuel process |
Producing IPK biojet from bio-isobutanol involves three sequential steps:
- Dehydration of the renewable isobutanol to isobutylene
- Oligomerization of the isobutylene to mostly trimers/tetramers to produce C12 and C16 molecules
- 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.
| Fig. 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 applications.
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 operations, etc.
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 scoping evaluation.
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 allowed.
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.
| Fig. 5. Biojet plant financial summary analysis. |
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 traditional methods.
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.
| Fig. 6. Bio-isobutanol to paraxylene, gasoline |
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. HP
|The authors |
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.