Recent Advances in Biodesulfurization (BDS) of Diesel Fuel
By Michael Pacheco, Energy BioSystems Corp.
Overview (back to top)
Energy BioSystems Corp. (EBC) is developing a unique refinery process using bacteria to selectively remove sulfur from diesel and diesel blend stocks. The process operates at ambient temperature and pressure and uses air instead of hydrogen to remove the sulfur. The biocatalyst for the process has been genetically engineered to have a high level of enzymatic activity for the selective oxidation of sulfur in diesel fuel and to produce a water-soluble organic sulfonate product.
The feedstock to EBC's new process can be any refinery diesel stream that contains a relatively high concentration of polynuclear aromatic sulfur heterocyclic compounds. The process is suitable for treating LCCO (light cycle oil), LCSGO (coker distillate), HS-HMD or the product of a diesel hydrotreater. The process can be used instead of conventional hydrodesulfurization (HDS), but it also provides considerable synergy with HDS. Upstream of an HDS unit, the technology can produce a very profitable yield of the organic sulfonate commodity and improve the performance of the downstream diesel hydrotreater. Downstream of an HDS unit, the technology removes the residual dibenzothiophenes left behind by the hydrotreater and provides a low-sulfur product with improved lubricity and oxidation stability.
Recent advances in BDS process technology are described and illustrated with results from a wide variety of refinery diesel streams. Examples of surfactant products from diesel BDS are included, along with limited information on process economics. Finally, the status of EBC's first commercial BDS project in Valdez, AK, is briefly reviewed.
The Sulfur Penalty (back to top)
The environmental driver for diesel sulfur reduction is well-established. Sulfur is the key diesel fuel property that has to be changed to allow auto manufacturers to achieve the Tier 2 vehicle emission reduction targets of the U. S. Environmental Protection Agency (EPA). In fact, there is widespread belief that if fuel sulfur levels are reduced in order to enable efficient after-treatment, the after-treatment device will become the primary driver on tailpipe emissions and all other fuel properties will have only minor or secondary effects. This is according to a recent Society of Automotive Engineers (SAE) review paper [1].
Meeting sulfur regulations on petroleum products is driving up the cost of refining, because conventional hydrodesulfurization becomes increasingly expensive and less efficient in handling sulfur removal as lower and lower sulfur levels are reached [2, 3] At last fall's SAE International Fuel & Lubes meeting, at least one refiner estimated that the potential cost to U.S. refiners of investing in low-sulfur diesel could reach $30 billion [4]. Biodesulfurization, which focuses on sulfur removal and doesn't necessarily attack other aromatics, is expected to provide refiners worldwide a cost-effective method of meeting new lower-sulfur standards while lowering energy consumption and, thus, CO2 emissions.
Biocatalytic desulfurization, or biodesulfurization, is a proprietary process based on naturally occurring aerobic bacteria that can remove organically bound sulfur in sulfur heterocycles of petroleum without degrading the fuel value of the hydrocarbon matrix. Enzymes in the bacteria can selectively oxidize the sulfur, then cleave carbon-sulfur bonds. BDS operates at ambient temperatures and atmospheric pressure, and so is expected to be significantly lower in cost than conventional hydrodesulfurization technology in achieving lower sulfur levels. Additionally, since the BDS process is oxidative, addition of hydrogen is not required, thus avoiding a significant element of conventional desulfurization operating costs. BDS can be used as a substitute for, or in conjunction with, existing HDS technology.
Biodesulfurization Basics (back to top)
The concept of microbial desulfurization of petroleum is not new; in fact, the first patents covering this technology in the U.S. were issued in 1948. Early attempts to use bacteria and enzymes to selectively remove sulfur from hydrocarbons failed primarily because of an inability to control the action of the bacteria, so although they desulfurized the oil, they also consumed much of the fuel in the process. In 1988, researchers at the Institute of Gas Technology (IGT) achieved a breakthrough in microbial desulfurization with the isolation of two unique strains of bacteria that could selectively remove sulfur from dibenzothiophene (DBT), the industry-recognized model for heterocyclic sulfur molecules found in petroleum [5]. U.S. patents were issued on these two bacterial strains in 1992 [6, 7].
In 1991, Energy BioSystems Corp. (EBC) obtained exclusive, worldwide rights to IGT's desulfurization technology and in 1992 achieved a critical milestone when the relevant genes from the patented bacteria (IGTS8) were cloned and sequenced. Dr. Daniel Monticello reported on these early achievements when he addressed the NPRA Annual Meeting in 1993 [8]. Since that time, progress on the productivity of the biocatalyst, the development of an engineered bioreactor system, and byproduct disposition have been significant [9]. In EBC's new process, the biocatalyst is produced and regenerated within the battery limits of the BDS unit, providing lower catalyst cost and increased catalyst longevity. The new technology also produces high-value chemical products that can provide a substantial new revenue potential for desulfurizing low-quality, highly aromatic diesel components [10]. EBC is now in the advanced stage of developing a commercial refinery process for removing sulfur in the form of aromatic sulfinates from high-sulfur diesel and diesel-blend stocks, while also continuing research on biodesulfurization of other feeds, including crude oil and gasoline.
The Dibenzothiophene Pathway (back to top)
In biodesulfurization, heterocyclic sulfur compounds such as dibenzothiophene (DBT) are selectively oxidized in a two-phase (oil/water) bioreactor through a multi-step enzymatic pathway that produces water-soluble sulfur byproducts without degrading the hydrocarbon matrix. DBT is oxidized to a sulfinic acid and, in the last step, to ortho phenyl phenol and sodium sulfate.
It is now understood that a small quantity of reducing equivalents such as glucose, ethanol or the diesel fuel itself must be consumed by the catalyst to regenerate the cofactor and carry out the oxidation process, and achieve high sustained rates of BDS [11]. It is important to note at this point that the same enzymes that are used in the DBT pathway provide analogous pathways for other sulfur compounds such as mercaptan, sulfide, disulfide, thiophene, and benzothiophenes. The DBT pathway has been chosen as the model to discuss/illustrate BDS at least in part because HDS is very slow with DBT and alkylated DBT.
Barriers to commercial use: The question then becomes, why isn't BDS being used today? One barrier to commercialization to this point has been that the natural concentration of desulfurization enzymes in the native bacteria originally discovered in the soil (IGTS8) was so small that the catalyst was too slow and the activity too low for an economical BDS process. Additionally, IGTS8 was not effective for a broad range of sulfur types. EBC has been working since 1992 toward improving the biocatalyst and process technologies to overcome these barriers.
Since 1990 researchers at EBC have increased the catalyst increased activity level more than 200-fold. The most recent advance in catalyst activity has come from optimizing media in the bioreactor itself. As a result of these advances, activity has been improved more than two orders of magnitude, and is well within one order of magnitude of the level required for a cost-effective BDS process.
An order of magnitude improvement has been achieved since 1996 via a combination of catalyst and process advances. Longevity has been increased more than tenfold and is at a commercially acceptable level for straight-run and hydrotreated feeds.
Selectivity: The most recent advances in EBC's biocatalyst technology have occurred within the past 12 months. Selectivity issues are being addressed and resolved. EBC resumed screening of soil isolates and also initiated an aggressive program of directed evolution and gene shuffling to discover or produce new enzymes that are capable of removing a broader range of sulfur types from petroleum. The overall goal of this research is to broaden selectivity to the point where the technology can achieve the full extent of desulfurization required by refiners, i.e., reduce diesel sulfur to less than 50 ppm. To this end, EBC scientists are building a library of catalysts with broader selectivity than IGTS8.
New opportunity: Additionally, EBC researchers discovered that the enzymes offered the potential to produce high-value chemical products. Sulfones which concentrate in oil, and sulfinates and/or sulfonates which concentrate in water can be selectively produced. Any of these oxidized products are easily recoverable and have much greater chemical value than elemental sulfur produced by HDS, and are even more valuable than the diesel fuel itself.
Biocatalytic Process Development (back to top)
The biocatalytic process design (i.e., reactor and separations) forms the basis that sets the requirements on catalyst activity and longevity needed to ensure that capital and operating costs for BDS will be lower than for HDS. The original BDS process design basis placed a very high requirement on catalyst activity. Following a 1997 reassessment, EBC engineers decided to dramatically change reactor design, separations, and catalyst regeneration within the battery limits of the BDS process (ISBL).
Reactor Design: Engineering advances in the area of reactor design have focused on increasing volumetric reaction rate, reducing cost/volume to build reactors, and reducing cost to operate and maintain reactors. The rate of reaction was enhanced by changing the operating conditions, while the cost to build, operate, and maintain the reactor was reduced by changing the mechanical design.
Increased volumetric reaction rate: Average volumetric reaction rate was increased by staging the reactor design instead of working with a single stirred tank reactor to achieve deep desulfurization. This permits greater fractional conversion for a given total reactor volume. Reactor staging maximizes the driving force for reaction kinetics and minimizes the effects of product inhibition.
Reactor design has also been instrumental in custom-tailoring the reactor design specifications and operating conditions to suit the physiology of the catalyst. The current reactor design utilizes a much lower water-to-oil ratio than previous designs; this change not only reduced the reactor size, but also forced changes to be made in the downstream equipment used to separate the emulsion. Typical results comparing how reaction rate responds to increased catalyst concentration are shown in Figure 1.
EBC's new process design takes advantage of higher catalyst density and higher reaction rates in the bioreactors.
Reduced cost per unit volume to build reactors: Reducing the cost/volume to build reactors was achieved by changing from mechanically agitated reactors to air-lift designs. Eliminating mixers from the reactors reduced the cost to operate and maintain the BDS unit. Agitators in designs described in EBC literature prior to 1998 were not only large capital-cost items, but also accounted for a large operating expense for electrical power, and a very significant maintenance cost. The physiology of our current prototype catalyst, combined with the high catalyst concentration and lower water-to-oil ratio used in the current reactor design, ensures sufficient oil/water interfacial surface area for good liquid/liquid interphase transport.
Primary Separations: There are several fundamental criteria governing the design of the primary separation section of the BDS process. The separations must be able to recover the treated oil product (desulfurized diesel) from the reactor effluent; recycle the active biocatalyst to the reactor; and recover sulfur-bearing chemical products, either from the water in the case of sulfates, sulfinates or sulfonates, or from the oil in the case of sulfones. The process must also be able to achieve the separations cost-effectively, utilize unit operations that are suitable for a sanitary biological process, and allow the reactor to operate at the optimum cell density, as determined by the knee in the upper curve in Figure 1.
The difficulty of separation increases with increasing catalyst concentration in the reactor effluent, as particle-stabilized emulsions are formed. Early separation schemes employed by EBC were limited to not more than 6 gm catalyst/L. A new separation scheme has been invented that capitalizes on the benefits of the emulsion phenomena in the reactor and still achieves all the separations criteria. EBC's new pilot plant in The Woodlands uses the new separation scheme, which is not fully disclosed herein; however, a few characteristics can be described:
- the current design does not involve any centrifuges;
- the only pieces of rotating equipment are pumps;
- the separation objectives are performed in two discrete stages;
- added chemicals are required to temporarily destabilize the emulsion; and
- construction, operation, and maintenance costs are dramatically lower than designs described in EBC literature prior to 1998.
Catalyst Regeneration EBC's early process designs (pre-1994) assumed that the catalyst did not have to be alive. Later designs (1995-1996) involved a living catalyst produced off-site. The logistics of sanitary handling, shipment, storage and use of living bacterial cells are problematic. In late 1997, EBC scientists and engineers discovered that is was possible to produce the catalyst within the bioreactor and also achieve significantly longer catalyst life by proper choice of catalyst and operating conditions. This is one of the most prominent improvements in our BDS process design basis, as it enables the biocatalyst to maintain its catalytic activity much longer than previous designs. The improvement in longevity is illustrated in Figure 2; process integration in Figure 3.
The new unit, designed to process 3 gpd, replaced a larger (5 bbl/d) pilot plant in St. Louis which was dismantled in May 1998.
Development of High-Valued Product Applications (back to top)
EBC's most advanced catalysts offer the potential to produce a variety of high-value chemical products from the sulfones, sulfinates, and sulfonates that can be produced and recovered from the BDS process. A few commercially relevant prototypes have been demonstrated. Examples of two of these - hydrotropes and surfactants - are mentioned here.
Hydrotropes: A hydrotrope is defined as an additive that increases the water solubility of another compound. Examples of commercial hydrotropes include sodium xylene sulfonate, sodium cumene sulfonate, and sodium toluene sulfonate.
Surfactants: Linear alkyl benzene sulfonate (LAS) is widely recognized as a workhorse in the anionic surfactant industry. About 4 billion pounds of LAS are produced annually. A variety of derivatives have been made via oxidation and chemical derivatization of the sulfinate product, CxHPBS, derived from BDS of real diesel fuels. Derivatization consists of either direct alkylation of the unsulfonated ring or chemical reactions involving the OH group. Many of these derivatives display surface activity very similar to LAS. One such derivative is illustrated in Figure 4, where the molecular structure is illustrated. The reduction in surface tension and critical micelle concentration are very similar to LAS.
Integration of BDS into the Petrochemical Industry (back to top)
The hydrotropes and LAS products described above are already products of the petrochemical industry. The benzene can come from reformate. Olefins are usually produced from kerosene-derived paraffins, but alpha olefins can also be used. The overall integration of refining and hydrotrope/surfactant manufacture is illustrated in Figure 5.
Hydrotropes and surfactants are just one specific set of chemical targets for integrated refining and petrochemical manufacture from BDS. Other possible chemical applications are possible with this process because of the selective oxidation-extraction that biocatalysis provides. Examples include phenolic resins, adhesives and biocides.
State-of-the-Art BDS Results with Refinery Diesel Streams (back to top)
Capabilities of EBC's biocatalytic process are evolving very quickly. Results for several diesel boiling range feedstocks illustrate the current state of the art..
Case 1—BDS Downstream of HDS:Case 1, illustrated by Figure 6 represents a situation in which BDS is used downstream of HDS. The feed for this example is a hydrotreated diesel fuel blend containing 20% LCCO. Achievable depth of desulfurization (%DES) is somewhat insensitive to the absolute level of sulfur, as current catalysts are able to remove 65-70% of sulfur from diesel blends that have already been hydrotreated below 2000 ppm. This configuration permits minimization of H2 consumption by using BDS to remove refractory DBT compounds after moderate HDS. The low CO2 emissions and low energy requirements of BDS are very advantageous in all applications of diesel BDS.
Case 2—BDS in Place of HDS: Case 2, illustrated by Figure 7 represents a situation in which BDS is used instead of HDS. In this case, the feed is virgin medium-sulfur diesel. Achievable depth of desulfurization is more variable for virgin fuels than for processed fuels. The range of desulfurization achievable with current catalysts is in the range of 40-70%, dependent on K-value and endpoint. More aromatic diesel fuels and virgin diesels with lower endpoints are more deeply desulfurized. This application permits desulfurization of diesel fuel without the investment in H2 supply, Claus unit, and tail gas unit required by HDS.
Case 3—BDS of High-Sulfur Cracked Stocks: Case 3, illustrated by Figure 8 represents a situation in which BDS is applied directly to high-sulfur cracked stocks, as might be valuable in a complex refinery where a goal is to maximize chemical production and maximize the utilization of existing HDS assets to achieve deep desulfurization of diesel fuel. The feed for this example is HS-LCCO. As can be seen from this example, the highest levels of sulfur removal are achieved with cracked stocks (LCCO, coker, visbreaker). Typically 75-90% of the sulfur can be removed from cracked stocks, including LCCO and coker distillate (LCSGO). This application also reduces the size of the BDS unit, relative to Case 1. EBC has experience with HS-LCCO with sulfur contents ranging from 1.5 to 3.3 wt% sulfur. The achievable percent desulfurization is always above 75%. The highest levels of BDS have been achieved with LCSGO.
BDS Process Economics (back to top)
Scoping economics for the BDS process have been estimated using the SuperPro Designer simulation software tool developed for the biotech industry by Intelligen, Inc. This flexible modeling tool was calibrated to match a detailed cost estimate prepared for one of EBC's clients, and is used by EBC engineers to screen process improvements based upon economic impact. A summary of the cost estimates from this software for the three feedstock cases shown in the section above are tabulated in Table 1. These estimates assumed an improvement in activity above the current level. Also, some improvement in substrate range was assumed.
| Case 1 | Case 2 | Case 3 | |
|---|---|---|---|
| Size | 30 MBPD | 10 MBPD | 20 MBPD |
| Feed | hydrotreated blend 80% SRGO, 20% LCCO |
Straight-run diesel | LCCO |
| Sulfer | |||
| —Feed | 500 ppm | 0.5 wt% | 2.0 wt% |
| —Product | 50 ppm | 500 ppm | 0.2 wt% |
| Capital, ISBL | $27 MM | $17 MM | $60 MM |
| Sulfinate Production (MM lb/year |
10.6 | 34.3 | 254 |
The results of the simulation indicate that BDS is competitive with HDS in certain applications and may be more cost-effective in certain Case 2 and Case 3 applications, depending on the value of the sulfonate product. Moreover, BDS is clearly still in a stage of rapid improvement in technical capability. The evolution of BDS technology and a qualitative summary of the impact that each phase of this evolution has had on process economics is illustrated in Figure 9.
Overall Advantages of BDS Technology (back to top)
- Safety and simplicity: BDS operates at ambient temperatures and pressure, and there are no hazardous chemicals or catalysts used in the process.
- Avoids lubricity problems of deep diesel HDS: Biotreated diesel displays no decrease in lubricity, as observed with deep HDS [13, 14]. Instead, the lubricity is consistently improved by biotreating, as measured by either the SBOCLE test or the RFRR test methods.
- Low CO2 emission and energy consumption: It has been estimated that BDS results in about 70-80% lower CO2 emitted and/or energy consumed per barrel as compared with HDS [15].
Options for PNA and S reductions: Sulfur is the key to emissions, and it is very clear that sub-50-ppm S will be required to meet future regulations. Other fuel specification changes appear to have no significant impact on emissions; however, if aromatics are to be lowered, it appears PNA's will be the target. BDS can reduce sulfur and convert sulfur-bearing PNA's to valuable chemicals.
What to Expect in the Future (back to top)
BDS is expected to be available as the lowest-cost option to achieve deep diesel desulfurization targets. All that is needed is a modest increase in catalyst activity (and a small improvement in extent of desulfurization in some cases). BDS technology has many intrinsic advantages over more severe HDS processing, and an increased drive toward environmentally sound processes and products will favor use of biocatalysis and biocatalytically produced chemical products in large commodity applications. In the more distant future, BDS is expected to be available to upgrade other refinery streams, including gasoline, crude oil, heavy fuel oils, FCC feeds, etc. Biologically catalyzed reactions could include viscosity reduction, biocracking, metals reduction, and other processes.
Commercialization Status (back to top)
As of the writing of this report (January 1999), EBC and its alliance partner Kellogg, Brown & Root were preparing the basic process design basis for the first commercial licensee to build a diesel BDS unit. The licensee is Petro Star Refining, and the unit is planned for construction at the company's refinery in Valdez, Alaska. Target for the unit to be operational is third quarter, 2001. The Petro Star process installation is expected to be the first commercial scale demonstration of diesel biodesulfurization. In addition to upgrading the refinery's 5 MBPD diesel stream, this plant will produce approximately 10 MM lbs/year of the new aromatic sulfonate commodity, which will be available for commercial use.
