Ecofining New Process for Green Diesel Production from Vegetable

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Franco Baldiraghi, Marco Di Stanislao, Giovanni Faraci, Carlo Perego, Terry Marker, Chris Gosling, Peter Kokayeff, Tom Kalnes, and Rich Marinangeli

Introduction

Existing technology for producing diesel fuel from vegetable oil has largely centered on the production of FAME biodiesel [1-3]. While FAME has many desirable qualities, such as high cetane, there are other issues associated with its use such as poor stability and high solvency, leading to filter plugging problems. Moreover, there has been little integration of renewable fuels production within petroleum refineries to date, despite the rapidly increasing growth in renewable fuels demand. The segregated production of renewable fuel components increases cost, since the existing infrastructure for distribution and production of petroleum-based fuels is not utilized. In addition, about the 9 vol.% of the product is glycerol, which is a low value product in unrefined form and has a limited market when refined. Methanol is required as a co-feed and feedstocks containing high concentrations of fatty acid can cause operational problems due to saponification reaction with the caustic present as a catalyst [4].

For all of these reasons, UOP LLC and Eni S.p.A. recognized the need for different processing route to convert vegetable oils into a high quality diesel fuel or diesel blend stock that is fully compatible with petroleum derived diesel fuel. The two companies started a collaborative research effort in 2005 to develop such a process based on conventional hydroprocessing technology that is already widely deployed in refineries and utilizes the existing refinery infrastructure and fuels distribution system. The result of this effort is the UOP/Eni Ecofining process. This new technology utilizes widely available vegetable oil feedstocks to produce a high cetane, low gravity, aromatics- and sulfur-free diesel fuel. The cold flow properties of the fuel can be adjusted over a wide range to meet various cloud point specifications in either the neat or blended fuel.

The main improvement of the Ecofining technology compared to the conventional FAME biodiesel is that it allows refiners to obtain a synthetic fuel that has a similar chemical composition and similar chemical-physical properties to petroleum diesel. For this reason the product can all be easily blended with conventional refinery products. Moreover, the integrated production of the green diesel allows the refiner to

Table 8.1 Green diesel fuel properties versus mineral ULSD and conventional biodiesel (FAME).

Ultra low sulfur diesel (ULSD)

Biodiesel (FAME)

Green diesel

Oxygen content (%)

0

11

0

Specific gravity

0.84

0.88

0.78

Sulfur content (ppm)

<10

<1

<1

Heating value (MJkg-1)

43

38

44

Cloud point (°C)

0

-5 to 15

-20 to 20

Distillation range (° C)

200-360

340-370

200-320

Polyaromatics (wt%)

11

0

0

NOx emission (wt%)

Baseline

+10%

-10%

Cetane number

51

50 , 65

70 , 90

Stability

Baseline

Poor

Baseline

control the quality of the renewable blending products. In addition, all of the Ecofining by-products are already present during normal refinery operation and do not require any special handling. Table 8.1 compares the properties of green diesel fuel with those of mineral ultra low sulfur diesel (ULSD) and of FAME biodiesel.

From Table 8.1 the evident advantages of green diesel advantages over mineral diesel fuels and FAME are:

• Green diesel is a high quality cetane component (CN > 80), which means higher engine efficiency.

• Green diesel is a hydrocarbon mixture, not an oxygenated organic compound, which means it has the same energy content as mineral diesel fuel and higher than FAME.

• Green diesel is a stable blending component without double bonds and oxygenated molecules.

• Green diesel maintains car reliability and reduces distribution costs.

• Green diesel low density is an advantage over FAME because it allows to upgrade low valuable and high density refinery streams, thereby expanding the diesel pool.

• Green diesel has the same boiling range as a mineral one. This prevents vaporization problems in the combustion chamber and it does not impact on the boiling point specification in case of blending with mineral diesel fuel.

• Green diesel is produced by a "refinery" process that permits quality control of biofuel and the use of existing infrastructure and fuel distribution systems.

• Green diesel meets the highest requirements of car manufacturers and can be utilized with all diesel automotives without modification.

From Vegetable Oil to Green Diesel

In general, the processing of biologically derived feedstocks is complicated by the fact that these materials contain a significant amount of oxygen. The feedstocks of

Green Diesel Production
Figure 8.1 General chemical structure of triglycerides.

primary interest in the Ecofining process are primarily vegetable oils such as soybean, palm, jatropha or rapeseed (including canola) oils. Other products such as animal fats and greases can also be used as a feedstock. Vegetable oils mainly consist of triglycerides with typically 1-2% free fatty acid content. Figure 8.1 shows the chemical structure of triglyceride molecule.

Triglycerides and fatty free acids both contain relatively long, linear aliphatic hydrocarbon chains. A fatty acid is a carboxylic acid, often with a long unbranched aliphatic tail (chain), which is either saturated or unsaturated. The aliphatic hydrocarbon always contains an even number ofcarbon atoms and also corresponds to the carbon number range typically found in the range of the diesel fuels. The triglyceride molecule has a three-carbon "backbone." The general chemical formula is RCOO-CH2CH(-OOCR')CH2-OOCR", where R, R', and R" are long alkyl chains. The three fatty acids RCOOH, R'COOH and R''COOH can be all the same (R = R' = R''), or all different (R „ R' „ R''), or only two the same (R = R' „ R''). Chain lengths of the fatty acids in naturally occurring triglycerides can vary, but 16,18 and 20 carbons are the most common. Most natural fats contain a complex mixture of individual triglycerides; because of this, they melt over a broad range of temperatures.

As an example of the complexity of the vegetable oil composition, the main properties of soybean, rapeseed and palm oil are reported in Table 8.2. In particular, the concentration ofthe fatty acid type, the chain length and the olefin bonds in each chain are indicated. Moreover, in the last column the same characteristics for crude rapeseed oil are reported. Table 8.2 shows that the vegetable oils can differ from in the amount of oxygen, and the fatty acid distribution as well as in the degree of unsaturation. As far as contaminants are concerned the vegetable oils can contain sulfur, phosphorus, alkali metals and free fatty acids. The amount of these contaminants is strongly affected by the purity grade of vegetable oil.

The quality of vegetable oil, in particular the fatty acid distribution and the degree of unsaturation can affect the properties of FAME biodiesel, but they do not affect the properties of green diesel.

Table 8.2 Main properties and composition of soybean, palm and rapeseed.

Vegetable oil

Soy

Palm

Rapeseed

Rapeseed

Form

Refined

Refined

Refined

Crude

Carbon (%)

77.46

76.93

77.97

78.86

Hydrogen (%)

11.23

11.60

11.42

11.85

Oxygen (%)

11.31

11.47

10.61

9.29

Nitrogen (ppm)

6.8

6.3

3.1

25

Sulfur (ppm)

<3

<3

<3

4

Phosphorus (ppm)

<5

<5

<5

100-600

Alkali metals (ppm)

10-15

10-15

10-15

100-600

Acid number (mg-KOH g_1)

0.3

0.5

0.11

2.3

Specific gravity at 15 °C

0.923

0.916

0.921

0.922

Fatty acid distribution

Palmitic acid (16-0)a (%)

12.50

34.30

6.70

6.70

Stearic acid (18-0)a (%)

0.65

1.63

1.45

1.45

Oleic acid (18-1) (%)

27.81

43.93

54.76

54.76

Linoleic acid (18-2)a (%)

54.19

14.27

27.51

27.51

Linolenic acid (18-3)a (%)

4.67

4.51

7.76

7.76

Iodine number

117-143

35-61

94-120

aNumbers in parentheses refer to the chain length followed by the number of double bonds.

aNumbers in parentheses refer to the chain length followed by the number of double bonds.

Renewable Diesel Process Diagram
Figure 8.2 Vegetable oil hydroprocessing alternatives.

As stated in the introduction, the main objective of this new technology is to remove all the oxygen from the aliphatic chain. Two options have been investigated to produce green diesel from renewable feedstocks: co-processing in an existing distillate hydroprocessing unit or building a dedicated unit [5]. Figure 8.2 gives a block diagram for the two alternative configurations.

Figure 8.3 Vegetable oil co-feeding lowers HDS activity for LCO: at the same average reactor temperature (WABT) the desul-furization activity is lower.

Figure 8.3 Vegetable oil co-feeding lowers HDS activity for LCO: at the same average reactor temperature (WABT) the desul-furization activity is lower.

The co-processing route was initially evaluated as an attractive option, since the existing equipment could be re-used, but after careful evaluation it was recognized that the co-feeding can cause several design problems and some limitations to the final products. These reasons can be summarized as follows:

• catalyst HDS activity reduction due to the high oxygen content of the vegetable oil with the effect of reducing the catalyst life;

• the cold flow property issue will limit the volume of vegetable oil that can be co-processed because in the co-feeding process the vegetable oil is converted into «.-paraffins having poor cold properties.

This is evident in Figure 8.3, which shows the results of a co-feeding pilot plant test using LCO (light cycle oil) + 50% vegetable oil as feedstock. The extent of vegetable oil effect in the co-feeding process depends on the amount of vegetable oil and on the type of fossil diesel.

Other problems of a co-processing operation are:

• need to add quench to the existing reactors to control temperature rise due to exothermic biovegetable oil hydrotreatment;

• revamp of the recycle H2 system to account for CO, CO2 and H2O production;

• upgrade the metallurgy to handle fatty acids;

• addition of a pre-treatment reactor to handle phosphorus and the alkali metals.

For these reasons, it was evaluated as more cost-effective to implement the process in a dedicated unit optimized for renewable feedstocks to overcome the problems listed above. Therefore, the Ecofining technology is a stand alone process carried out in a two-stage process. In the first stage the triglyceride structure of vegetable oil is cracked, removing the oxygen and saturating the double bonds, while in the second stage the cold flow properties are adjusted according to market requirements.

The most valuable way to target the first specification is to use hydroprocessing technology. This route uses hydrogen to remove the oxygen from the triglyceride molecules. The oxygen is easily removed via three competing reactions: hydrodeoxy-genation, decarbonylation and decarboxylation. The three-carbon "backbone" yields propane that can be recovered easily when the process is integrated into a refinery. The oxygen contained in the feed is removed from the fatty acid chain either as CO/ CO2 or water. In addition, all olefinic bonds are saturated, resulting in a product consisting of only n-paraffins. The extent of each reaction depends on the catalyst and process conditions and the global mechanism can be depicted as:

RCH3 + 2H2O

RCOO - CH2CH(-OOCR')CH2-OOCR" C3H8 + R'H + CO + H2O

From this scheme it is evident that the hydrodeoxygenation produces a paraffin having the same number of carbon atoms as the fatty acid present in the triglycerides structure, while decarbonylation and decarboxylation produce a paraffin having one carbon atom less than the fatty acid present in the triglycerides.

A bimetallic hydrotreating catalyst, tailored for this kind of feedstock, has been developed and tested under different conditions and the main results are summarized here.

According to the experimental investigation the overall reaction is highly exothermic (e.g., the AH of reaction depends on the in-saturation of the fatty acids in the triglycerides and can vary from 97kcalkg-1 for stearic acid and 422kcalkg-1 for linolenic acid) and very fast. A complete conversion of vegetable oil has been observed for a short contact time and moderate temperature (e.g., above 310 °C). Increasing the temperature will affect the decarboxylation reaction while increasing the pressure will affect hydrodeoxygenation. Figure 8.4 shows the ratio of the decarboxylated chain (C17) to the hydrodeoxygenated one (C18) with varying temperature for three different pressures. The hydrogen consumption depends on the type of feed and is proportional to the concentration ofthe olefin bonds and oxygen content in the triglycerides. Notably, the liquid products are only n-paraffin with a 99% volume yield while the byproducts are mainly in the gas phase, made up of CO/CO2 and H2O deriving from the oxygen ofthe fatty acid and propane from the three-carbon "backbone" present in the triglycerides structure.

Despite the high cetane number, the high cloud point ofthe liquid stream coming out from the hydrotreating reactor has a great impact in limiting the volume that can be blended with mineral diesel. To overcome this restriction these paraffinic streams have to be isomerized in a second stage. For such a purpose an appropriate hydroisomerization catalyst, based on a precious metal loaded on a mild acidic carrier, has been utilized. The reaction occurs at mild operating conditions and the kinetic behavior can be described according to the kinetic model reported for n-C16 [6], after proper parameter estimation.

Calendrier Remplir

Temperature, C

Figure 8.4 Ratio of decarboxylated to hydrodeoxygenated chain versus reactor temperature at three different pressures.

Green Diesel Production
ISO paraffin/NORM paraffin wt/wt Figure 8.5 Green diesel cloud point as a function of the iso-/n-paraffin ratio.

The scope of this second stage is to control the cold flow properties of the final green diesel. As explained in the open literature [7-9], the diesel yield from the process will depend on the severity required in the isomerization reactor to meet cold flow specifications.

Figure 8.5 reports the correlation between the green diesel cloud point versus the ratio iso-/n-paraffin. Notably, the increase in iso-/n-paraffin ratio is obtained by increasing the severity of the process. The latter results in a different product distribution (diesel, kero and naphtha), with the diesel cut being the larger part (e.g., diesel 88-99 vol.%). Even under more severe conditions the diesel produced has a very high cetane number (>75) and contains no aromatics.

Extensive performance testing has been carried out in pilot plants to determine the optimum process conditions, catalyst stability and product properties. A range of vegetable oils have been processed in the pilot plants, including soybean, rapeseed, palm and jatropha oil. Other potential feedstocks, including tallow and greases derived from animals, have been evaluated.

As far as catalyst stability is concerned, a long pilot plant test has been carried out using soy oil and the results (Figure 8.6) show very good stability and product selectivity during the first 2000 hours of stream.

According to its superior properties, green diesel is a premium blending component. To verify this, the blending of mineral diesel with green diesel and engine tests have been performed. The diesel blends produced fulfill the highest requirements of car manufacturers and can be utilized with all diesel automotives without modification. Reduced tailpipe exhaust emissions were proven, and it was discovered that green diesel can be blended up to 65% to European 10ppm diesel fuel. Additionally, low-value hydrotreated LCO can be blended with green diesel to be introduced into the typical diesel pool, meeting the European diesel specification.

UOP/Eni Ecofining Process

According to the above results an integrated process called the Ecofining process has been developed. Figure 8.7 shows a simplified flow diagram.

In the Ecofining process, vegetable oil is combined with hydrogen, brought to reaction temperature, and is then sent to a reactor section where the vegetable oil is converted into the green diesel product. The reactor section can consist of either a deoxygenation reactor or a combination of a deoxygenation and isomerization reactors to achieve better cold flow properties in the green diesel product. The product is separated from the recycle gas in the separator and the liquid product sent to a fractionation section. The design of the fractionation section can vary from a one-column system producing on-specification diesel and unstabilized naphtha to a three-column system producing propane, naphtha and diesel products. It is envisaged that most installations will be a single column and the lighter products will be recovered in other existing refinery process units. The recycle gas is treated in an amine system to remove CO2. Table 8.3 gives typical product yields.

Green Diesel Production

J Reactor temperature -»-Diesel yield Figure 8.6 Catalyst stability testing: (a) hydrotreating catalyst; (b) isomerization catalyst.

J Reactor temperature -»-Diesel yield Figure 8.6 Catalyst stability testing: (a) hydrotreating catalyst; (b) isomerization catalyst.

Life Cycle Assessment

A life cycle analysis (LCA) of different diesel-production routes has been performed [10, 11].

The LCA evaluation program is a method to determine and compare the environmental impacts of alternative products or processes, including impacts of initial resource extraction to waste disposal. In this study, the scope ofthe analysis was from crude-oil extraction through combustion of the refined diesel fuels in a vehicle. It was assumed that all fuels had the same performance in the vehicle. The primary focus of

Green Diesel Production

Diesel Product

Figure 8.7 Simplified Ecofining process flow-scheme.

Table 8.3 Typical products yield of the Ecofining process

Feed

Vegetable oil (wt%) 96.5-97

Products

Diesel properties

Cetane number >80

the analysis was on fossil energy use and GHG emissions, although other impact categories are included.

Figures 8.8 and 8.9 summarize the results of the LCA. Green diesel compares favorably to biodiesel in the LCA study. Fossil energy consumption (Figure 8.8) over the life cycle is expected to be reduced by between 84-90% for green diesel produced from soybean oil or palm oil, respectively, when H2 is produced internally from byproducts (green diesel-B) rather than from fossil resources (green diesel-A). Thus, green diesel has the potential to displace more petroleum resources per energy content in the fuel compared to biodiesel. Larger reductions in greenhouse gas emissions for green diesel relative to biodiesel have been predicted by this study for soybean feedstocks (Figure 8.9), but lack of verifiable data on palm oil prevented any conclusions being made for this feedstock. Overcoming this omission and inclusion

Green Diesel Production
Figure 8.8 LCA analysis comparison of green diesel versus FAME biodiesel and a petroleum diesel: energy consumption per unit of diesel energy.
Green Diesel Production
Figure 8.9 LCA analysis comparison of green diesel vs. FAME biodiesel and a petroleum diesel: GHG emissions.

of other environmental impacts will be the subject of future research in green diesel production and use.

Conclusion

UOP and Eni have developed the Ecofining process, a new, sustainable, route for converting vegetable oil into premium quality diesel fuel. This green diesel product is a superior alternative to FAME, with significantly better diesel product properties and is fully compatible with conventional mineral diesel products. The Ecofining process is fully developed and available for licensing from UOP. Eni is currently evaluating the size and the location of the first commercial unit to be realized within one of its refineries.

References

1 Fangrui, M.A. and Milford, H. (1999) Bioresour. Technol., 70, 1.

3 Bournay, L., Casanave, D., Delfort, B., Hillion, G. and Chodorge, J.A. (2005) Catal. Today, 106, 190.

4 Lotero, E., Liu, Y., Lopez, D.E., Suwannakarn, K., Bruce, D.A. and Goodwin, J.G. Jr (2005) Ind. Eng. Chem. Res., 44, 5353.

5 Holmgren, J., Gosling, C., Marinangeli, R., Marker, T., Faraci, G. and Perego, C. (September 2007) Hydrocarb. Process, 86, 67.

6 Calemma, V., Peratello, S. and Perego, C. (2000) Appl. Catal. A-Gen., 190, 207.

7 Alvarez, F., Ribeiro, F.R., Perot, G., Thomazeau, C. and Guisnet, M. (1996) J. Catal., 162, 179.

8 Girgis, M.J. and Tsao, Y.P. (1996) Ind. Eng. Chem. Res., 35, 386.

10 Kalnes, T., Marker, T. and Shonnard, D.R. (2007) Int. J. Chem. Reactor Eng., 5, A48, pp. 1-9.

11 Holmgren, J., Gosling, C., Kokayeff, P., Faraci, G. and Perego, C. Green diesel production from vegetable oils. AICHE Spring Conference, April 200710-25.

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