Hydroprocessing technology

As already indicated in Section 1.3, transesterification is not the only process used to produce a biofuel from triglyceride feedstocks (vegetable oils, tree- borne oils and animal fats). A competing approach involves hydroprocessing, which is a generic term encompassing a number of catalytic processes using hydrogen, e. g. hydrogenation, hydrocracking and hydrotreating (Fig. 15.17). Hydroprocessing of natural lipids occurs in stages, from saturation of double bonds, stripping off the triglycerides attached to the fatty acid chains and removal of all other oxygen-containing compounds, to cracking with formation of a diesel fuel. Careful selection of the hydrogenation catalyst can result in a diesel fuel directly without the need for a separate hydrocracking step.

uring the last century the chemical industry has turned hydrocarbons into a variety of functionalised chemicals, mainly by catalytic oxidation (see Fig. 15.1). Preparation of under-functionalised renewable diesel fuels, essentially alkanes, from overfunctionalised vegetable oils by catalytic hydroprocessing

is essentially the opposite and chemically less appropriate. Yet, marrying refining and renewable fuels provides a potentially lucrative opportunity to create ‘ green’ fuel with attractive refining economics.

Developing petroleum-like fuels from renewable sources is an ingenious idea. An early process for simultaneous hydrogenation and cracking of vegetable and tree oils was developed at the Saskatchewan Research Council (SRC). The technology has been licensed to Arbokem, Vancouver (BC). One of the feedstocks successfully used in the development of a biofuel called ‘ SuperCetane’ is tall oil, a by-product from the Kraft pulping process from pine and Douglas fir. From an economic perspective the total cost of SuperCetane would range between 18 and 32 cents per litre (1998), as compared to 66 cents using vegetable oils [70]. Expired Canadian patents from the 1980s using tall oil, never commercialised, have indicated that hydrogenation of natural oils can result in very high cetane number diesel fuel of very high purity (e. g. zero sulphur and aromatics). US Patents No. 4,992,605 to Craig and Soveran [71] and No. 5,705,722 to Monnier et al. [72] disclose production of high cetane number additives for diesel fuels (mainly C15 to C18 paraffins) by hydroprocessing of biomass feedstock, selected from canola oil (CO), sunflower oil (SNO), rapeseed oil (RSO), palm oil (PMO) and fatty acid fractions of crude tall oil, at 623-723 K and at 4.8-15.2 MPa.

Recent renewed interest in vegetable oil hydroprocessing technology by the oil industry has probably been triggered by tax and other ‘breaks’ made available to the biofuels industry, as well as by the lack of sufficient product quality control by (part of) the biodiesel industry (see Section 12.4), especially with respect to oxidative stability in summer and cold-flow properties in winter. The oil industry, which supplies the dominant ultra-low sulphur diesel (ULSD) component of biodiesel blends, is in a position to supply a renewable diesel itself and probably with better product quality control than many small biodiesel producers. Almost any petroleum refiner that produces low-sulphur diesel fuel or gasoline has sufficient surplus hydrogen available to manufacture a hydrodiesel.

The biodiesel industry thus faces a new challenge in the growing interest by the oil industry in producing a consistent and reliable diesel fuel by hydrogenating the same natural feedstocks (vegetable oils and the like) to make a product variously called renewable synthetic diesel, green diesel, hydrodiesel, hydrotreated vegetable oil (HVO), hydrogenation-derived renewable diesel (HDRD), or bio-hydrogenated diesel (BHD). Petrodiesel — like fuels from agricultural lipids have sometimes also been referred to as ‘synthetic biodiesel’ or ‘second-generation biodiesel’ but these terms are misnomers as they do not meet the definition of biodiesel, which is, according to ASTM D 6751: ‘mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats’.

The projected manufacturing capacity of this new renewable diesel alternative (670 kt in 2006) is sizeable in biodiesel industry terms but is a very small product volume for any major oil refiner. However, if the petroleum industry seriously moves into hydrotreating of natural lipids, it is likely that the supply side (vegetable oil feedstock) will even more rapidly become the primary growth-limiting factor for both biodiesel and hydrodiesel. At the same time, it is equally likely that neither biodiesel produced by methanolysis of natural lipids nor hydrodiesel can shortly become mainstream energy sources, but they will be very attractive additives for ULSD fuel because of their important contributions to lubricity, engine performance and combustion emissions control. Since HVOs are hydrocarbons, they largely meet conventional diesel fuel requirements (EN 590, ASTM D 975, Worldwide Fuel Charter Category 4), but not ester specifications (EN 14214, ASTM D 6751).

Hydrotreated fuels present both advantages and disadvantages with respect to biodiesel, which are mainly on account of differences in chemical nature, namely saturated hydrocarbons vs. partially unsaturated fatty acid alkyl esters, respectively (Table 15.14). While vegetable oils with low saturation level are preferred for biodiesel applications, higher saturation in the oils is preferred for hydrodiesel in view of lower hydroprocessing costs. On the whole, renewable feedstock costs are comparable. Hydroprocessing of low-quality feedstocks (high FFA content) gives no problems. The synthetic diesel fuels resulting from natural lipid hydrotreatment have very attractive properties, including a high cetane number (no need for additives to meet fuel specifications), very low NOx emissions, and zero aldehyde emissions (a problem with biodiesel). The heating value is about 11% higher. As hydrodiesel is a fuel very similar to traditional petrodiesel, existing engines can be used without modifications.

Table 15.14 Characteristics of hydroprocessed bio-based alkane fuels

Advantages over biodiesel:

• Flexible cold-flow properties and better oxidative/storage stability (more fossil diesel-like)

• More combustible energy content per unit mass (11%)

• High cetane number ratings

• Lower NOx exhaust emissions

• Fewer obstacles in gaining acceptance by engine manufacturers

• Co-production of propane, naphtha and jet fuel

• No low-value liquid by-product (waste water, soaps)

• Require no special tank storage

• Can be sent by pipeline to blending sites or retail distribution centres Disadvantages over biodiesel:

• Less environmentally friendly in terms of CO and soot

• Poor fuel lubricity

• Slower biodegradability

• Poor safe handling properties (flammability, toxicity)

The low lubricity is easily (and inexpensively) corrected by means of lubricity additives. Hydrodiesel avoids the glycerol glut problem. However, many of the typical biodiesel advantages are lost when using vegetable oil — based alkane fuels. Among these are excellent fuel lubricity characteristics, safer handling properties due to being non-flammable, non-toxicity, much faster biodegradabilities and reduced exhaust emissions, including CO and particulate matter. By transforming vegetable oils into hydrocarbons all benefits linked to the biodegradability of the product are lost. The energy balance of hydrodiesel from biological sources is less favourable than that of biodiesel since its production requires higher T, p, higher investment and operating costs. Moreover, conversion of the feedstock is lower (<90%) as compared to biodiesel (98%). Consequently, the use of agricultural lipids for this purpose could be a waste of these feedstocks. Table 15.15 summarises the main features of hydroprocessing biodiesel/diesel substitute production methods. High investments are required for high-pressure hydrogenation units (7100 million per unit).

Hydroprocessing is a preferred refinery operation. Two alternative vegetable oil hydroprocessing routes can be considered, namely co-processing in an existing distillate hydroprocessing unit, or a stand-alone unit (Fig. 15.18). Problems connected with co-processing of vegetable oils are several:

• reduction of HDS catalyst activity (due to oxygen);

• high exothermicity of hydrotreatment of vegetable oils;

• co-production of CO, CO2 and H2O (impacting on H2 recycling); and

• need for a pre-treatment reactor (removal of Na, Ca, P and other metallic contaminants).

Co-processing tends to produce paraffinic diesel with a high cloud point and is of variable quality. This is because traditional refineries are not built to handle

Table 15.15 Hydroprocessing biodiesel/diesel substitute production methods

Advantages:

• Proven technology

• Allows processing of non-refined oils (TGs, FFAs)

• Feedstock flexibility

• No glycerol marketing problems

Disadvantages.

• High investment and production costs

• High T, moderate to high p

• Pre-treatment/post-treatment (isomerisation)

• Low reaction yield

• Eliminates beneficial properties (see Table 15.14)

• Catalyst coking, poisoning, regeneration

• By-products

CO2 and oxygenated feedstock, intrinsically associated with hydrotreatment. Therefore, it is better to have a dedicated facility for making green diesel. A renewable diesel unit is a separate unit integrated with an oil refinery, which uses existing logistics, energy, hydrogen, quality control laboratory and other facilities. A separate unit avoids diesel hydrotreater (DHT) catalyst life issues and increases flexibility. In a stand-alone unit there is no co-feed of the bio-component with crude oil. A stand-alone unit needs access to a hydrogen supply or a dedicated hydrogen plant.

Hydrogen required for HV O production can come from a variety of sources, including a conventional H2 plant, a typical naphtha-to-gasoline Platforming™ unit, or steam reforming of the light fuel by-products of the HVO process; the latter option decreases the overall fossil fuel consumption.

From a petroleum refiner’s perspective, a fully deoxygenated renewable diesel is considered a premium diesel blending component. The boiling range is comparable to typical diesel products, with substantially higher cetane and lower density. These are very valuable properties that enable refiners to optimise the amount of lower value refinery streams that can be blended into the refinery diesel pool while still meeting all of the required EU diesel specifications. Furthermore, cold-flow properties can be regulated by adjusting process conditions, thus making the process more flexible than biodiesel with respect to feedstock selection and plant location. In contrast to FAMEs, where fuel properties depend on feed origin and process configuration, paraffinic diesel products (NExBTL, Green Diesel, etc.) are independent of feed origin. A recent Canadian operability test of low-level
blends, namely B2 (CME; HDRD) in winter (T > 233 K) and B5 (CME/ TME = 3/1; HDRD) in summer was successful [73].

Life cycle GHG emissions of renewable diesel are 40-60% lower than for those of fossil diesel fuel. Yet, the total GHG values are comparable to FAME, since farming and feedstock production, including the use of fertilisers, limestone and other materials, are major contributors to GHG emissions. The renewable diesel process itself, including hydrogen production, makes only a minor contribution to GHG emissions. Life cycle analyses of hydrodiesel and biodiesel need further comparison, in particular in terms of energy efficiency and the effects of the hydrogen production process. The social impact of the geographically dispersed biodiesel production and of centralised bio-based alkane fuels production will be quite different, with potential benefits of energy crops for rural and urban communities versus large petroleum companies.

Several petroleum companies are now operating small incremental hydrogenation facilities at their refineries to produce hydrodiesel in dedicated or co-processing mode (Table 15.16). EU hydrodiesel capacity is under development in Finland, Ireland, Italy, Portugal, The Netherlands and Austria. Dedicated NExBTL and Ecofining™ facilities are located inside existing refineries. ConocoPhillips is focusing its resources on conversion technologies that will enable large-scale biofuel production. Current technologies seem likely to fail if their scale is expanded too greatly. Although ConocoPhillips buys biodiesel for its terminals as part of normal commercial activities, its technology development is entirely on second — and third-generation fuels, primarily through thermochemical pathways. The focus of the research is finding ways of making processes sustainable while reducing CO2 emissions and using cost-effective feedstocks. Target products are hydrocarbon fuels that will fit within the existing distribution system (pipelines and terminals)

Table 15.16 Vegetable oil hydrogenation facilities

Stakeholder

Technology

Feedstock

Product

SRC/Arbokem

Dedicated

Tall oil

SuperCetane

Neste Oil

Dedicated (NExBTL)

PMO, RSO, TLW

Renewable

UOP LLC/Eni

Dedicated (Ecofining™)

SBO, PMO, RSO

Synthetic Diesel Green Diesel

Petrobras

Co-processing (H-BIO)

SBO, CAS

Renewable diesel

ConocoPhillipsa

Co-processing

SBO

Hydrocarbon fuel

Tyson Renewable

Co-processing

TLW

Renewable diesel

Energyb

BP

Co-processing

TLW

Renewable diesel

Cetane Energy

Proprietory

Undisclosed

Renewable diesel

aWhitehead refinery (County Cork, Ireland).

bFull-scale commercial operations (175 MMgy) as from spring 2009.

and vehicles without retrofit. Infrastructure is considered as the key element in successfully implementing large-scale sustainable biofuels. ConocoPhillips has technologies that convert triglycerides into fuels and currently uses this technology position to make renewable diesel commercially for soybean oil and tallow. Activities comprise co-processing of SBO (Whitegate refinery, County Cork, Ireland; 50 kt/yr as from December 2006) and of TLW (Tyson Renewable Energy joint venture; 175 MMgy as from 2009). ConocoPhillips is also committed to renewable fuel from algae. Similarly, Petrobras (Brazil) has developed its proprietary H-BIO co-processing technology (see Section 15.4.3). In Australia, BP intends producing 110 kt/yr biofuel from animal fat at its Bulwer Island (Queensland) refinery by 2008 using proprietary co­processing technology that uses hydrogen to convert heated tallow (at least 50% of the feedstock). PTT/Toyota Thailand is developing bio-hydrogenated diesel (BHD).

Hydrodiesel is very similar to synthetic diesel obtained (at much higher cost) by converting carbonaceous feedstocks into syngas and finally into a hydrocarbon product using Fischer-Tropsch technology (see Section 15.5). Future processes (BTL technologies), which enable utilisation of residual biomass (cheaper feedstock) instead of lipids (expensive foodstock) to produce liquid hydrocarbons, are of great interest.

Cetane Energy LLC (Carlsbad, NM) has recently developed small-scale (3 MMgy) hydroprocessing technology.

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