octane gasoline and commodity petrochemicals. That all ... primarily from Canadian oil sands has also ramped up. ... is used to increase gasoline octane [13].
Chemical Reaction Engineering Challenges in the Refining and Petrochemical Industries – The Decade Ahead
Thomas F. Degnan, Jr. Department of Chemical and Biomolecular Engineering University of Notre Dame Notre Dame, IN 46556 e-mail: [email protected]
Abstract
An emergent “bifurcation” in hydrocarbon resource and product supply and demand will provide new challenges and opportunities for the refining and petrochemical industries. Many of these challenges and opportunities are likely to be addressed through advances in chemical reaction engineering.
Over the next decade, changes in hydrocarbon supply and demand will most assuredly drive technology advances in the refining and petrochemical sectors. This same assertion was certainly true in the mid-1900’s when growth in product demand drove the addition
of new refineries and new capacity. In the middle of the last century, the goals were advanced process technologies that scaled-up more effectively and produced more highoctane gasoline and commodity petrochemicals.
That all changed, as markets matured and projections of “peak oil” were increasingly heard the late 1980’s, 1990’s and at the beginning of the twenty-first century [1,2].
From 1980 to 2000, the number of U.S. petroleum refineries declined by 50%. However, the overall processing capability in terms of primary distillation capacity actually increased by ~10% (Figure 1). Smaller, less efficient refineries shut down, and the
existing larger, more efficient refineries, expanded to take their place. Refineries that were linked with petrochemical or lubricant plants had distinct advantages because of their product diversification and process integration. Advanced process control, improved models and process intensification facilitated debottlenecking. Technology improvements continue to allow fewer refineries to produce more gasoline. At the beginning of 2014 there were 142 refineries in the U.S. with a total refining capacity of 17.9 million barrels per day. This was one less refinery than at the beginning of 2013. Yet in 2013 U.S. refining capacity increased by 101,000 BBL/day [3].
Two additional refineries started up at the end of 2014, mainly as a result of increased crude and condensate production from the Bakken formation in North Dakota and the Eagle Ford formation in south Texas. These were the 20 kBD Dakota Prairie Refining LLC refinery in Dickenson, ND and the 50 kBD Kinder Morgan condensate processing facility near Houston, TX. These are the first new refineries constructed in the U.S. since 2008.
Today, gasoline demand along with more stringent environmental regulations and improved energy efficiency continue to drive most refinery planning. In Europe diesel rather than gasoline demand drives decisions since diesel continues to power the majority of Europe’s light duty vehicles. Globally, the outlook to 2040 is for diminished gasoline consumption, and for accelerated growth in demand for higher molecular weight fuels including diesel and jet fuel (Figure 2) [4]. A tightening of vehicle fuel economy standards coupled with reduced light passenger vehicle demand in developed economies, and an overall population shift toward large cities is likely to depress gasoline consumption.
Impact of Hydraulic Fracturing
Within the U.S., it is clear that broader application of hydraulic fracturing has dramatically changed both natural gas and crude supply. Over the past year, the growth
in petroleum production from unconventional resources has largely been responsible for a decline in the price of crude. Mainly as a result of the additional U.S. hydrocarbon production, global supply continues to exceeded demand. U.S. crude production has grown by 72% over the past five years, from 5.6 MBBL/day in 2010 to 9.3 MBBL/day today (Figure 3) [5].
This growth has been accompanied by a change in the overall crude composition. Petroleum from reservoirs such as Bakken and Eagle Ford is lighter, more paraffinic, and lower in sulfur and resid than the more conventional mid-continent crude (Figures 4 and 5a and 5b) [6,7,8]. At the same time, production of heavier, more asphaltenic crude primarily from Canadian oil sands has also ramped up.
Implications
The result of North American hydrofracturing and oil shale development has been a “bifurcation” in the crude composition. Proportionally, there is more light paraffinic condensate and more heavy residuum in today’s crude supply than ever before. This
“dumbbell” distribution poses both challenges and opportunities for a refining sector that has, over the past thirty years, geared up for an increasingly heavy slate.
Coincidentally, there is a similar bifurcation in product demand. Diesel consumption is projected to grow annually by 1% while gasoline demand is actually projected to decline by ~0.9% per year [5]. On the lighter end, basic chemicals volumes, led mainly by increased demand for light olefins and aromatics, are projected to grow by 2% annually. Lubricant base stock (Group I and II) capacity is expected to expand over the next 10 years by approximately 150,000 BBL/day. However, base oil consumption projections are for decreased demand in the lighter base socks and increased demand for gas-toliquid (GTL) and PAO based lubricants [9,10].
The compositional and volumetric changes in supply and demand have several implications for refining process:
•
Fluid catalytic cracking (FCC) units, which have been a mainstay for gasoline production, are being used to produce more propylene. Less propylene is being produced from conventional ethylene units as chemical manufacturers move toward ethane and away from naphtha as the feedstock for ethylene production. FCC’s are efficient for converting light paraffinic naphtha to propylene using ZSM-5 additives. Projections of increased propylene demand are also driving additions to propane dehydrogenation capacity [11].
•
Alkylation units (both HF and H2SO4) are or will be running near capacity as isobutane supply increases, butane prices decline, and demand for high-octane gasoline remains robust. As interest grows in adding alkylation capacity, there may be renewed interest in alternatives to the conventional hydrofluoric and sulfuric acid catalyzed processes. The HF and H2SO4 processes have proved to be very competitive economically to solid catalyzed alkylation processes, despite a substantial amount of work on the solid acid catalyzed processes [12]. So long as the refining industry maintains a safe and environmentally responsible record for operating these units, it is difficult to envision how the current technology will be replaced. The solid acid catalyzed processes under development (mainly zeolitic) will have to prove to be at least as durable, and will have to produce gasoline octane quality better than the highly evolved and optimized HF and H2SO4 catalyzed processes in order to gain a foothold. Of course, technology is not always the key motivator in changing processes. New legislation or environmental regulations may prompt refiners to move away from liquid acids.
•
Reforming units will be directed toward producing more petrochemical grade aromatics (BTX) as gasoline demand declines and alkylate, rather than reformate, is used to increase gasoline octane [13]
•
Hydrocracking and hydrotreating capacity will continue to expand, driven by greater demand for more diesel and lower sulfur diesel, higher quality lubricants, and more resid conversion. The projected long term increase in supplies of
natural gas and the decline in natural gas prices has led to a commensurate decline in the cost of hydrogen. This has also provided greater incentive for refiners to invest in expanding hydrotreating and hydrocracking capacity. Some of the capacity increases may result from debottlenecking or use of more active and selective catalysts, which will expand production without capital expense [14]. It is also expected that there will be additional hydroprocessing capacity required to treat biomass-derived stocks including fast pyrolysis liquids [15] as well as hydroisomerization of light naphtha for higher octane gasoline [6].
Representative of the shifts in refinery process technology development towards more petrochemical production are UOP’s PetroFCC TM and RxPro™ processes [16,17]. The PetroFCC TM process combines changes in FCC reactor and regenerator design with changes in process conditions to greatly enhance the production of light olefins and aromatics. Propylene yields of 12 to 16 wt% (vs. conventional 4 to 6 wt%) based on fresh feed can be achieved. The RxPro™ process makes use of a multi-stage reactor configuration together with reactor technology elements found in conventional FCC/RFCC designs to generate propylene yields in excess of 20 wt%. The process is useful for converting both vacuum gas oil (VGO) and petroleum resids. The aromatic byproduct from this process can be sent to an aromatic complex for additional benzene, toluene, and p-xylene production.
The most significant opportunities for technical advances are likely to lie in finding alternative uses for molecules that would have originally ended up in gasoline. In fact,
the most acute need is in developing catalysts and processes that convert the lighter gasoline-range paraffins into higher value chemicals, fuels, or lubricants.
Propane, butane, pentane and hexane conversion to higher value products is especially attractive for several reasons:
•
Light paraffins, which comprise the bulk of natural gas liquids (NGLs) or condensate, are being produced at much higher than historical rates as a result of hydrofracturing.
•
U.S. Government restrictions on gasoline volatility (Reid Vapor Pressure or RVP) are projected to tighten forcing more n- and isobutene out of the gasoline pool, particularly in the summer months and especially in the warmer southern and southwestern states.
•
Ethanol, now typically blended 10 vol% into gasoline in the U.S. per Reformulated Fuel Standard (RFS), may possibly increase to 15% in some parts of the U.S. as a result of changes in the RFS. The EPA has allowed 10% ethanol blends a 1-psi RVP waiver, but has not extended this waiver to 15 vol% ethanol blends. If 15 vol% ethanol blends become widespread, even more light hydrocarbons will be forced from the gasoline pool.
The most attractive opportunities for use of light paraffins lie in four areas:
1) Producing high quality (high cetane number) diesel range molecules with carbon numbers from C14 to C20. 2) Producing high viscosity index, highly paraffinic lubricants with carbon numbers from C24 to C100 3) Producing more light olefins (propylene, butenes, pentenes, and hexenes) that can be used as petrochemical building blocks for polymers, fuels, and specialty chemicals 4) Producing specialty oxygenates (alcohols, ethers, ketones, aldehydes, carboxylic acids, etc.), amines, thiols and mercaptans.
The opportunities are listed in descending order of prospective volume demand but increasing order of prospective value uplift.
Presently, there are only six commercial routes for upgrading light alkanes to higher value intermediates:
1) Dehydrogenation to produce the corresponding light olefins 2) Hydroisomerization of C4 to C7 n-paraffins to isoparaffins for production of higher octane gasoline 3) Alkylation with a light olefin, a route used commercially to produce gasoline range molecules from butanes and increasingly from pentanes. 4) Dehydrocyclization to produce primarily benzene, toluene, xylenes, and hydrogen. This is commercially practiced in UOP’s Cyclar process
5) Partial oxidation to produce syngas that can be further reacted to higher MW paraffins through the Fischer-Tropsch reaction or to methanol 6) Free radical cracking, typically conducted at higher temperatures, and most commonly to produce ethylene
Once the higher or lower molecular weight olefins, aromatics, and synthesis gas intermediates are produced, it is a rather simple matter technically to convert these to the targeted molecules. The challenge then reverts to improving selectivity and improving separation efficiency.
It is difficult to improve economically upon conventional isoparaffin-olefin alkylation for building more valuable, higher molecular weight alkanes. However, the reaction is stoichiometric, reacting only one paraffin molecule for every olefin molecule. The challenge lies in activating light paraffins alone or in a ratio of more than one paraffin per olefin to produce higher molecular weight compounds. Oxidative dehydrogenation to olefins and selective oxidation to functionalized monomers are both areas that have received a significant amount of attention in the past. Both routes generally suffer from poor selectivity because of complicated reaction networks that produce CO and CO2 as a result of consecutive reactions involving deep oxidation of intermediates.
Similar to UOP’s development of its PetroFCC TM and RxPro™ processes, future advances are almost certain to require some combination of novel reactor design, novel catalytic materials, and perhaps atypical process conditions. For all of the work that has
been directed towards activating methane to produce higher molecular weight products, it is worth asking whether any of the same approaches or lessons learned can be applied to propane, butanes, pentanes or hexanes? Clearly, there is incentive to improve upon current light paraffin activation routes [18].
Bifurcation – Trends and Challenges
In addition to the previously described “bifurcation” of hydrocarbon resources (feedstocks) and products, we are also confronting a “bifurcation” in manufacturing (Figure 6). Refineries in highly developed economies are increasing in complexity.
Many refiners are adding or expanding petrochemical and lubricant capabilities or are attempting to improve energy efficiency via addition or expansion of cogeneration units [19]. More stringent environmental regulations are prompting refiners to add systems such as low NOx burners in their furnaces and selective catalytic reduction (SCR) units for their FCC units and furnaces. Advances in process control are allowing refiners to handle more complexity with the same number of highly trained operating staff.
At the same time, smaller and simpler refineries such as the Dakota Prairie Refining and Kinder Morgan condensate processing refineries mentioned above are being constructed in both highly developed and less developed economies. It is reasonable to ask whether the optimal refining strategy of the future is either small, simple, and part of a widely distributed fuels-petrochemical-energy production network or large, complex, and part of a highly concentrated network? There are good arguments for both strategies.
In either case, the need for advanced chemical reactor technology will be strong. There is a good case to be made for more and more efficient hydroprocessing units to convert the heavier, more asphaltenic crudes as well as to hydroisomerize the more highly paraffinic unconventional crudes. We should expect FCC reactor and regenerator technology to continue to evolve with the need to convert more light naphtha into olefins and aromatic and heavier, more refractory heavy vacuum gas oil (HVGO) and resids into low sulfur gasoline and diesel.
Moving bed reactor technology, such as that used in UOP’s CCR units or Axens Eluxyl units should continue to evolve not only for production of aromatics and high-octane gasoline, but also for reactions such as light paraffin dehydrogenation (e.g. UOP’s Oleflex) and perhaps for light paraffin dehydrocyclizaion (e.g., UOP’s Cyclar).
The ‘bifurcated” technical challenges (Figure 7) are threefold:
1. Develop process technology to more efficiently build-up molecular weight from the lighter hydrocarbons derived from condensate and unconventional crudes while also developing process technology to more efficiently reduce the molecular weight of the heavier crudes derived from oil sands and heavier Middle Eastern crudes. It remains a major challenge, for example, to redirect the FCC unit design away from its original “gasoline production” focus to one where it produces more diesel range fuels from the heavier feedstocks. Will we ultimately be able to develop reactor technologies that have the versatility to process a broader range of feedstocks?
2. Identify and develop more efficient technologies for separating light hydrocarbons from, for example CO2, which is sometimes found in large concentrations in natural gas reserves while at the same time finding ways to more efficiently separate heavier more asphaltenic and higher metals resid molecules from lighter, less asphaltenic, lower metal resid molecules. Cryogenic separation of light paraffins and olefins is still among the most energy intensive processes in the refinery. Distillation remains the largest energy consumer in the refinery. The more than 40,000 distillation columns in North America consume about 40% of the total energy used to operate plants in the refining and bulk chemical industries [20]. Recent strides in the development of inorganic membranes (e.g., zeolitic and metal organic framework (MOFs)) as well as durable polymeric membranes may provide the keys. Using process
intensification strategies, will we see more technologies that allow simultaneous reaction and separation for processes?
3. Continue to progress the predictive and simulation capabilities of more sophisticated models, such as the Structure-Oriented Lumping (SOL) compositional models developed by ExxonMobil [21], while simultaneously developing and improving upon simple, intuitive learning models necessary for helping operators and less experienced or new engineers understand the fundamentals of chemical reactor engineering. For example, will it be possible to combine computational fluid dynamics (CFD) models with compositional reaction models to improve reactor design and operation?
We must also continue to develop a new generation of experts in reaction engineering. We need to do this while continuing to draw upon the experience of engineers who were involved in the development and commercialization of process technologies that we benefit from today. The demographics of the petroleum and petrochemical industries are challenging. According to several recent studies the US petroleum and petrochemical industry is heavily reliant on its older professionals. Nearly sixty percent are 50 years old and over creating a worker replacement ratio of 0.25. Stated differently, there is only one young professional for every four professional approaching retirement. This is cause for great concern. We need a new generation of talented young professionals to take over
from the retiring workforce that built this industry. [22,23] The same holds true for industrial chemical reactor engineering expertise. Diminished university research funding, which is increasingly focused on less mature, more topical chemical engineering subjects, is part of the challenge. However, it is important to continue to emphasize the career opportunities especially for those who combine knowledge of reaction engineering and catalysis.
Conclusions
Changes in the hydrocarbon resource base, brought about by new extraction technologies including hydraulic fracturing and oil sand extraction, present many new challenges and opportunities for the refining and petrochemical industries. There are new opportunities for light paraffin upgrading especially into high valued petrochemicals, but also into higher molecular weight fuels, and even lubricants. There are also opportunities for conversion and hydrogen enrichment of the heavier resid-like molecules into fuels other than gasoline. The future technology developments will invariably come from combining new chemical reactor designs with new catalysts or unconventional reaction conditions. Major technical challenges reside in three areas: (1) improved approaches for controlling hydrocarbon molecular weight through chemistries that both increase and reduce molecular weight, (2) more selective separation of both low molecular weight and high molecular weight hydrocarbons, and (3) development of both more complex and simpler predictive mathematical models. Industry and academia must work together to ensure
effective transfer of knowledge and expertise from a cohort of retiring chemical reaction engineers to the next generation of experts.
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