In Part 1 of this article, economic incentives were estimated for relaxing the requirement that biocrude entering the refinery infrastructure be oxygen (O2)-free. It was concluded that an ac-curate estimate of these incentives is not possible without a sig-nificant amount of additional data. Part 2 examines key issues that must be addressed and the associated data needed for this constraint to be relaxed.
Knowns and unknowns at the biocrude-refinery interface. In considering a feedstock as radically different as pyrolysis oil (pyoil), the refiner is faced with several unattract-ive knowns and a large number of unknowns. These factors rep-resent potentially serious threats to smooth refinery operation.
Raw pyoil from fast pyrolysis has a very high concentration of O2 (35%–40%), an element that is not found in crude oil. Its presence in raw pyoil causes material to be un-distillable due to its poor thermal stability, to be highly corrosive (TAN > 100), to have a high density (specific gravity is 1.2–1.3) and to be im-miscible in petroleum. Clearly, raw fast pyrolysis pyoil is a non-starter as a refinery feedstock. The question for the biofuels in-dustry then becomes: What would render pyoil admissible as a feedstock for refiners?
To answer this question, pyoil manufacturers must develop a better understanding of how refiners view the risks of admit-ting their materials, and what test data might persuade them to include partially upgraded pyoil (PUP) in their feed slates.
Considering PUP as a feedstock. Refiners thoroughly test every new feedstock before running it. In the case of PUP, a number of issues must be addressed before a refiner is willing to risk its equipment. Refiners have little or no experience with biomass O2 molecules other than ethanol. They lack both oper-ating history and a database for determining how PUP impacts processes and products. Consequently, refiners will proceed with great caution and require a significant amount of data be-fore running a new feed.
When running PUP, refiners will process it as a dilute blend in petroleum. Refiners have found that blending is a useful tool for handling difficult or acidic feedstocks. For example, high-acid crudes such as Chad (total acid number, or TAN, of approximately 5) can be successfully refined if diluted with low-TAN crudes. Typically, refiners target a blended-feed TAN
of less than 0.6; this is the common limit set by refineries that have not invested in metallurgical upgrades.1
Refiners are likely to resort to PUP blending for two rea-sons. First, the refiner will initially introduce PUP in very di-lute blends for risk management purposes. Over the longer term, the limited availability of PUP will set an upper limit on possible blends. Depending on the outlook, the amount of PUP that will ultimately be produced could limit blends with petroleum to a range of 3%–10%.2,3 For purposes of this article, it is assumed that PUP will be processed in dilute blends with low-TAN crudes.
Several of PUP’s physical properties are of concern, based on their unattractive characteristics in raw pyoil. These proper-ties are distillablility, corrosivity, heat exchanger fouling poten-tial, solubility, density and viscosity. PUP must be distillable;
raw pyoil is not. Refining’s primary step involves segregating the feed into specific boiling ranges that target the refinery’s end products. This process allows customized upgrading by fraction in specialized units downstream of the distillation tow-ers. If PUP cannot be distilled, then refiners cannot procure the fractions needed to produce on-specification end products.
Each of these fractions also must not cause corrosion or heat exchanger fouling. Making sure this does not happen involves an understanding of the composition of PUP’s residual O2 and how it is distributed by fraction. Feedstock corrosivity is important because most refinery equipment contains carbon steel, which is not particularly corrosion-resistant. Refiners must exercise great caution to ensure that their feedstocks are not very acidic.
Mistakes in this area can require expensive repairs during refinery turnarounds, and even cause units to fail. Similarly, fouling can disrupt heat management and unit operating con-ditions, leading to unplanned shutdowns. Heat exchangers are especially vulnerable; their heat-transfer dynamics can trigger fouling reactions by unstable feedstocks. Refiners know that raw pyoil is extraordinarily unstable. Solubility is important be-cause refiners almost always blend different feedstocks to keep units running at full capacity and to optimize process paths.
Feedstocks that are not soluble cannot be blended, which dis-rupts optimization.
The density of raw pyoil is very high relative to petroleum (specific gravity > 1.2) and represents an operational problem for a refiner. Oil-water separations take place at several points
throughout a refinery. These separations depend on the density of the oil being less than water. Finally, viscosity is important because each refinery has design viscosity limits on its pumping and handling equipment.
The next area of concern is the potential impact of the re-sidual O2 molecules in PUP blends on the performance of the various conversion and upgrading units. Many of these units use sensitive catalysts. Changes in the yields and product qual-ity from these units can significantly impact refinery economics.
Any changes in catalyst activity and life could negatively impact the refinery’s operability and overall service factor. All of this critical information must be determined in extensive pilot plant experimentation. A large number of processes are impacted (isomerization, reforming, catalytic cracking, hydrocracking and multiple hydrotreaters), so this represents a significant effort.
Finally, the presence of any biomass O2 molecules that sur-vive refinery processing to find their way into end products will prompt concerns about their impact on product quality. This will be particularly true if any PUP is directly blended into final products without catalytic upgrading. Extensive product qual-ity experimental work, possibly involving end-use testing, may be required to resolve uncertainties in this area.
In short, refiners will demand diverse and convincing data before seriously considering PUP as a refinery feed. To add fur-ther complexity, the above information will need to be available for pyoils generated using different biomass feed sources and for each of the different pyoil technologies that might find their way to commercialization. Collectively, these requirements rep-resent a significant hurdle constraining PUP’s development as a refinery feed blend component.
Upgrading pyoil: Knowns and unknowns. Pyoil’s proper-ties improve as O2 is removed, and the improvement is strongly correlated with the amount of oxygen removal. Therefore, re-sidual O2 can be used as a proxy for quality when discussing pyoil as a refinery feedstock.
At present, two routes are used for O2 removal: Catalytic fast pyrolysis (CFP), which removes O2 as carbon oxides; and hydrotreatment, which removes O2 as water. O2 removal via CFP suffers large yield losses when very low levels of residual O2 are targeted. Therefore, severe hydrotreating has become the preferred route for reducing O2 to very low residual levels (< 1% O2). Severely hydrotreated pyoil resembles a light sweet crude oil, making it a potentially attractive refinery feedstock.
In fact, a very low O2 product from the KiOR pyrolysis plant in Mississippi has been successfully coprocessed with petroleum by Chevron at its Pascagoula refinery.4 Unfortunately, achiev-ing O2 extinction via hydrotreating is very expensive.
As noted, the incentive for the introduction of PUP into a refinery is significant. It is now appropriate to assess how much information is available on the important areas raised in the preceding sections, and to determine the extent to which new data must be generated. As a general summary, the available data is very limited; the need for new data is extensive. The ex-isting data suffer because the size of most experimental equip-ment is too small to generate high-quality data or to produce the quantities of materials required for the requisite tests. The sample demands for work in this area will be substantial, which presents a significant barrier to obtaining the relevant data.
Refinery hardware integrity and operability. The most important physical properties of any refinery feedstock are its distillability, solubility in petroleum, corrosivity, heat exchang-er fouling potential, density and viscosity. Othexchang-er than den-sity and viscoden-sity, only limited information exists about these properties as a function of O2 level, particularly in the low-O2
concentration range. The limited data that does exist is based primarily on fast pyrolysis liquids.
Distillability. Raw pyoil is thermally unstable, rendering it impossible to distill. Given that distillation is a fundamental re-fining tool, finding the point where PUP can be distilled is very important. This point is not precisely known, but it appears to occur somewhere below 8% O2. This assumption is based on a brief scoping study conducted in 2010 by the US National Renewable Energy Laboratory (NREL), where batch distilla-tion of an 8.2% O2 sample was accomplished, but with some difficulty, due to its high water content.5 (Batch distillation of a 4.9% O2 PUP sample was easily carried out.) It is not accu-rately known how much lower the O2 needs to be for successful continuous distillation (i.e., no fouling in the distillation feed heat exchanger train).
As noted, determining the point at which PUP becomes dis-tillable is of economic importance. The catalysts and operating conditions for the conversion and upgrading processes have evolved over time and are quite diverse. A key driver here is the preservation of valuable liquids vs. losses to light gases. In other words, the processing severity required to convert the heavy fractions would, if applied to light fractions, lead to too much conversion to light gases and the unacceptable loss of more valu-able liquids. In applying this broad concept to PUP, there may be significant incentives to distill full-range PUP material at the dis-tillability threshold into a few individual boiling ranges, and then upgrade these in customized hydrotreaters operating at different severities. This contrasts with the existing approach of upgrad-ing all of the material in a supgrad-ingle reactor operatupgrad-ing at the severity set by the molecules that are most difficult to de-oxygenate.
Petroleum solubility. Raw pyoil from fast pyrolysis is not soluble in petroleum; this limits pyoil as a feed blendstock.
Vague information exists about pyoil solubility as a function of O2 concentration. For example, raw pyoil is only around 4%
soluble in benzene, whereas benzene solubility increases to ap-proximately 30% when the O2 content is reduced to around 20%.6 One company successfully solubilized a 7.1% O2 fast py-rolysis pyoil in vacuum gasoil.1 In summary, PUP’s exact solu-bility threshold in petroleum is not well defined, although it appears to be somewhere above 7% O2.
Corrosion. Given raw pyoil’s highly acidic nature, under-standing PUP’s corrosion potential is an area of high concern to the refiner. When discussing corrosion with refiners, some background is useful.
To protect their equipment from unacceptable corrosion, refiners have built up a large base of data and experience in dealing with acidic crudes. Unfortunately, a comparable base of experience and data does not exist for PUP’s carboxylic and phenolic acids. A database that ties actual corrosion rates at typical refining conditions to a simple acidity test is needed for PUP material. A modified TAN test (mTAN7) can provide more useful information about the acids in PUP material and is, therefore, the recommended simple acidity test for this work.
Most pyoil corrosion information found in literature is for raw fast pyrolysis pyoil at ambient conditions; this is of limited interest to the refiner, which is more concerned with high-tem-perature/high-pressure operating conditions. Not surprisingly, these raw pyoil corrosion studies indicate high corrosion rates at 94°C for the steel commonly found in refineries.8
A 2005 study used TAN measurements to conclude that PUP upgraded to 1%–5% might be introduced into a refinery if blended with low-TAN crudes.1 This conclusion is based on three assumptions:
1. PUP with a given level of TAN will show the same corrosion characteristics as petroleum
2. TAN blends linearly with dilution
3. TAN limit established for petroleum applies equally to PUP blends.
All of these assumptions require experimental verification.
Corrosion data of more interest to the refiner is beginning to appear in literature.9 Work done at Oak Ridge National Labs indicates that 3.3% O2 PUP shows negligible corrosion at 50°C, and PUP with < 0.5% O2 shows negligible corrosion up to 350°C, even for carbon steel.
Note: These studies were performed for a full-boiling-range pyoil. Refiners will be more interested in corrosion behavior as a function of boiling points and operating conditions. Limited information is available regarding how PUP acidity is distribut-ed by boiling point. Studies at Utah State on fractional catalytic pyrolysis found that acids tend to concentrate in the lower boil-ing fractions.10 Another crude assay analysis on a 4.9% O2 whole pyoil sample showed no measureable TAN for the 360°F+ frac-tions.6 These results directionally support the notion that, with regard to corrosion, some pyoil fractions may not need treat-ment to O2 extinction before entering the refinery.
Heat exchanger fouling. Given that raw fast pyoil is ther-mally unstable, it is to be expected that this material would quickly foul refinery heat exchangers. However, no literature information was found on this important subject. The com-plete information vacuum on PUP heat exchanger fouling is a major barrier to refiners seriously considering this material.
Here again, this information is needed as a function of O2 level and for typical refining cuts and operating conditions.
Density. The density of raw pyoil is very high relative to pe-troleum (standard gravity > 1.2) and represents an operational problem for refiners. Therefore, it is important to determine the point at which PUP density becomes less than 1. Fortu-nately, data on this topic exist, and it indicates that a tipping point occurs at or near 15%–20% residual O2.6
Viscosity. Fast pyoil displays unusual viscosity behavior as a function of O2 removal.11 Viscosity significantly increases to very high levels (50,000 cps–200,000 cps at 22°C–34°C) as O2
is reduced to 20%–25%; then, it falls with further O2 removal to < 100 cps at 22°C–34°C at < 10% O2. Consequently, it ap-pears that PUP upgraded to below 10% O2 would have accept-able viscosity from a refinery handling standpoint.12 Also, there is fairly good definition of the effect of O2 concentration on viscosity in the 1%–10% O2 region.
Summary of data needs on physical properties. As shown above, limited literature data on PUP exist with regard to several physical properties that are important to refiners.
It appears that levels of PUP O2 in the range of 1%–10% are worth further study. In this range, both viscosity and density do not appear to be limiting.
For PUP materials in this range, the refiner’s data needs include:
• Determining the precise PUP O2 thresholds for distillability and petroleum solubility
• Developing data on corrosion and heat exchanger fouling as a function of the key refinery boiling range cuts and O2 level for the range of 1% pyoil O2 up to the lower of the previously identified thresholds. This corrosion and fouling data should be developed at the typical operating conditions encountered by these cuts.
The reader is reminded that the impact of biomass feed and pyoil manufacturing technology on these studies must be measured. It is recommended that O2 speciation data be collected throughout these studies to allow for the building of a more fundamental understanding of PUP upgrading chem-istry. This should allow identification of improved upgrading approaches by:
• Identifying the specific carboxylic/phenolic acids and other contaminants found in PUP
• Observing where those acids and contaminants cluster by boiling point at different overall residual O2 levels
• Identifying which of these compounds are “bad actors”
from a refining standpoint, and determining when and how they are eliminated in the upgrading process.
The result of this work would be a first definition of “refinery-safe” fractions of PUP that can potentially be blended with petroleum for processing.
Performance of PUP blends in refinery processes. We next turn to the question of how diluted PUP blends impact key refinery conversion processes. Here, the refiner is inter-ested in the impacts on process unit yields and product quali-ties. Negative impacts on catalyst activity and catalyst life are also of concern.
Available information. Refining process data on pyoil blends are very limited and exist primarily for fast pyrolysis whole pyoil. Virtually no published data were found on re-forming or isomerization of light pyoil fractions. These refin-ing processes improve the quality of light streams for blendrefin-ing into gasoline. At the other end of the boiling range spectrum, no resid conversion data were found; this is not a surprise, since the amount of resid material in PUP is very low.
While a large amount of work has been done on hydrotreat-ing whole pyoil, little data exist on PUP’s performance and product characteristics when treated under the milder con-ditions of existing refinery hydrotreaters. FIG. 1 illustrates the significant differences between the operating conditions of typical refining hydrotreaters compared to pyrolysis liquids upgraders. Refiners will be concerned about their hydrotreat-ers’ ability to extract the last increments of PUP O2, which are more difficult to remove.
Crude assays are a key tool for evaluating a potential feed-stock. No published crude assays on PUP are available, and only limited crude assay data exist for fully upgraded pyoil.9 The available refinery process-related information is limited to hydrocracking and catalytic cracking.
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