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glycerol mixtures
Chemical Engineering & Processing: Process Intensification 142 (2019) 107553
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Canola oil/glycerol mixtures in a continously operated FCC pilot plant and comparison with vacuum gas oil/glycerol mixtures
T
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Marco Büchelea, , Matthias Swobodaa, Alexander Reichholda, Wolfgang Hoferb a b
Vienna University of Technology, Institute for Chemical, Environmental & Bioscience Engineering (ICEBE), Getreidemarkt 9/166-3, 1060 Vienna, Austria OMV Marketing & Refining GmbH, Trabrennstraße 6-8, 1020 Wien, Austria
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluid catalytic cracking Glycerol VGO Canola oil Refinery technology
Catalytic cracking of glycerol/canola oil mixtures was conducted in a continuously operated FCC pilot plant at TU Wien. The range of mixtures was from pure canola oil to pure glycerol with several mixtures in between. The aim of this research was to prove if the addition of glycerol to the feed can be feasible and to determine changes in product quality. The set riser temperature was 550 °C and the selected feedrate was 2.7 kg/h. As bed material a zeolite-based commercially available FCC-equilibrium catalyst was used. Utilizing a 7 lump model the products were characterized. Viable products like cracking gas (C1–C4) and the gasoline lump (paraffins, olefins and aromatics) were analyzed in detail. The experiments showed that product distribution was mainly dependent on the glycerol content in the feed. The gas, gasoline and LCO + residue lump decreased dramatically. Non-viable products like water, coke and carbon oxides, on the other hand, increased significantly. Hence, it was concluded that under set conditions and parameters glycerol might only be beneficial to the FCC process in small admixtures because of possible control coke-make. Additionally, a comparison was made between experiments using canola oil/glycerol mixtures and VGO/glycerol mixtures.
1. Introduction Even though there are concerns regarding the exhaust fumes of diesel engines and their harmfulness to health, in particular NOx, diesel engine cars are still widely used in many parts of Europe. Especially in countries like Austria, Spain or France diesel engine cars are still a dominant factor regarding market share [1,2]. On a global perspective, the usage of diesel/gasoil is expected to at least stagnate on a high level with rising demand in developing countries and due to new regulations in the marine bunker sector [3]. Additionally, strict environmental regulations and the promotion of using higher quantities of environmental sustainable feedstock through legislation makes it challenging for refineries in the EU to satisfy the demand for middle distillates [4]. In Austria the widely used method of replacing fossil fuel is the addition of so called Biodiesel (FAME – fatty acid methyl ester). The addition of which is allowed up to the amount of 7 v% in Diesel so far [5]. Hence, the production of Biodiesel surged to fulfil the political target values. On a global perspective biodiesel production grew by around 23 percent annually between the years 2005 and 2015. This corresponds to a sevenfold increase in only a decade [6]. The main production process for biodiesel is the transesterification
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of vegetable oils. There, a plant based oil like canola oil is esterified to FAME and the by-product glycerol using a base or acid as a catalyst [7]. Consequently, the leaping production of FAME leads to an exponential growth of glycerol production. This glycerol flooded the market and glycerol prices collapsed since demand remained stable [8]. Even price drops down to 10% of the original level were observed [9]. To resolve this issue, new applications for glycerol needed to be found. Therefore, the possible usage of glycerol as a cheap feedstock for fuel production was researched. One possible way is the co-processing of glycerol with other feedstocks in the fluid catalytic cracking (FCC) process which leads to this work. The FCC-process is one of the core processes in a refinery which basics were developed in the 1930s [10]. The basic principle behind the process is the cracking of low value high-boiling hydrocarbons into smaller molecules utilizing a catalyst and high temperatures. The valuable products produced are olefins (like ethylene and propylene), high-octane gasoline and to a smaller amount light cycle oil (LCO). Standard feeds for the catalytic cracking unit are normally gas oils from vacuum distillation units (VGO) but also heavy oil residues [11]. In general the FCC process is a very versatile and robust conversion process regarding feedstocks where different product spectrums can be obtained. This has been proven by various studies that utilized different
Corresponding author. E-mail address: [email protected] (M. Büchele).
https://doi.org/10.1016/j.cep.2019.107553 Received 8 February 2019; Received in revised form 7 May 2019; Accepted 30 May 2019 Available online 04 June 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
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feeds like e.g. vegetable oils or oils derived from pyrolysis [12–18]. Therefore, due to its flexibility and its establishment as one of the core refinery processes the FCC process is predestined to test alternative, more sustainable feedstocks. However, since industrial FCC plants produce around the clock and every production issue means money loss, feedstock tests on pilot plants are necessary to minimize economic risk for the industrial plant owner and prove viability. The objective of this work is to co-process glycerol/canola oil mixtures in an FCC pilot plant and to compare the results with another series of experimental runs of glycerol/vacuum gas oil mixtures conducted at the same pilot plant. At Vienna University of Technology (TU Wien) an FCC pilot plant has been developed by Hofbauer and Reichhold which was designed as an internally circulating fluidized bed system [19]. Later a second pilot plant was built by Bielansky and Reichhold which can process higher feedrates and introduced several improvements build in compared to the first plant [20]. Both plants show a high flexibility regarding process parameters (mainly riser temperature and feedrate) and can be operated with different feedstocks. Additionally, results of these pilot plants can be conveyed to industrial plants. This was shown in past cooperations with oil and gas companies when under similar setups (riser temperature and used catalyst) results of the pilot plant where compared with corresponding industrial FCC units.
Table 1 Key data. Pressure
Atmospheric
Regenerator temperature Riser temperature Total height Riser height Riser diameter Regenerator diameter Catalyst mass Feed rate Riser residence time Particle circulation rate C/O – ratio
500–800 °C 400–700 °C 3.2 m 2.5 m 0.0215 m 0.33 m 45 – 65 kg 2 – 8 kg/h ∼1 s 0.5 – 5 kg/min 10 – 50
Fig. 1. The corresponding key data is listed in Table 1. The plant is designed as an internally circulating fluidized bed system which results in a compact design. However, a side-effect of this construction type is the thermal coupling of the 2 plant sections: reactor and regenerator. The feed is pumped to a tubular oven using a gear pump where it is pre-heated. From there it flows through the feed inlet pipe into the riser where it evaporates immediately as soon as it gets in contact with the hot catalyst particles. This results in an increase in volume and thus the feed is transported pneumatically together with the catalyst particles to the top while cracking reactions occur. The residence time in the reactor is about 1 s. During the cracking reactions coke is generated which deposits on the catalyst particles and therefore, inactivates them. Once the catalyst-gas-mixture reaches the particle separator the spent catalyst is removed from the gas stream. The product gas leaves the plant at the top and is burned in a flare. A small portion of the product gas stream bypasses the torch and flows into a condensation apparatus where the products are gathered for analysis and separated into liquid and gaseous phase. The coke-loaded particles descend in a return flow pipe through the syphon into the regenerator section. Here, the particle bed is fluidized with air and coke deposits are combusted to provide the energy needed for the endothermal cracking reactions. The regenerated particles are then passing through the cooler system towards the bottom where they act again as a catalyst for the cracking reaction. This cycle provides a continuous plant operation. Except for the regenerator, all plant sections are fluidized with nitrogen which is especially important for the syphon and the bottom fluidization. Both act as a gas barrier that prevents oxygen from entering the product stream and product gas entering the regenerator. Additionally, the syphon acts as a stripper that removes remains of product gas from the spent particles. Parameters like regenerator temperature, riser temperature, particle circulation rate and feed rate can be adjusted in the ranges mentioned in Table 1. Depending on the chosen parameters the total residence time of feed in the plant is between 20–80 s.
2. Material and methods 2.1. Pilot plant A schematic of the FCC pilot plant located at TU Wien is shown in
2.2. Catalyst and feedstock The used FCC equilibrium catalyst is a zeolite – based catalyst developed for enhanced olefin and gasoline production. Its fluid dynamical properties were studied in advance to optimize plant operation. The minimal fluidization velocity was found to be 1.195 mm/s at 620 °C (Fig. 2). The catalysts particle size distribution is shown in Fig. 3. The mode, which defines the maximum of the particle size density distribution, is 82 μm. The feed utilized for the experiments consisted of glycerol and canola oil. In total 5 pilot plant runs were conducted where 3 different mixtures where fed into the plant with additionally one run with both pure glycerol and canola oil. Canola oil is a plant based oil which consists mainly of triglycerides with one glycerol molecule bound to 3 fatty acids. The hot filtrated canola oil was obtained from Rapso GmbH (Aschach, Austria). Some
Fig. 1. Pilot plant. 2
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Fig. 2. Pressure drop as a function of superficial velocity at 620 °C.
selected properties and the composition of fatty acids of canola oil are shown in Table 2. The glycerol utilized in this research is from the company Wasser Hygiene Chemie GmbH (Hilgertshausen-Tandern, Germany). It is solely plant-based and fulfils the quality requirements of the pharmaceutical and food industry. Some additional data is listed in Table 3.
Table 2 Fatty acid composition and physical and chemical properties of canola oil.
2.3. Analysis Due to the wide variety of hydrocarbons produced in cracking reactions an analysis model had to be developed that offers high accuracy with reasonable effort. This was solved using a so called lump model that is also used in industrial refining processes. Details of this 7 lump model are described in Table 4. The product gas is divided into a gaseous and liquid fraction utilizing a 3 stage condensation apparatus. The gained gaseous phase (not liquid at room temperature) is then analyzed in a gas chromatograph (SHIMADZU GC 17-A). Here, the gaseous fraction is studied using 2 separate columns and detectors. The gas lump (hydrocarbons C1 – C4) are analyzed using a FID and a Varian CP-Al2O3/Na2SO4 column. The N2 content is measured with a TCD and a CP CarboPLOT P7 column. The carbon oxide lump (CO, CO2) is measured online with a Rosemount NGA 2000 MLT3 of the company Emerson, an infrared gas analyzer. The liquid fraction consists of 2 phases, an aqueous and an organic one which are split up using a separation funnel. The aqueous phase that makes up the water lump is measured gravimetrically. The organic phase, on the other hand, is analyzed in a Simulated Distillation (SimDist) utilizing a second gas chromatograph. This GC is equipped with a Zebron ZB-1 column and a flame ionization detector (FID). Further information can be found in the supplementary data section. In a SimDist the organic phase is divided into 3 categories: gasoline, LCO and residue. Here, the separation criteria is the boiling range (Table 4).
Fatty acid composition Palmitic acid C16:0 Stearic acid C18:0 Oleic acid C18:1 Linoleic acid C18:2 Linolenic acid C18:3 Other
4.5 wt% 1.8 wt% 60.8 wt% 18.5 wt% 8.0 wt% 6.4 wt%
Physical and Chemical Properties Viscosity (40 °C) Density (20 °C) Bromine number Acid number Oxygen content Water content
35.9 mm²/s 915 kg/m³ 69.1 g Br/100 g 0.1 mg KOH / g 10.9 wt% < 0.1 wt%
Table 3 Glycerol properties. Density Purity Oxygen content Water content
1.26 kg/m³ > 99.5 wt% 52.1 wt% < 0.5 wt%
Additionally, a manual atmospheric distillation was conducted to gain a gasoline lump for further analysis. This gasoline lump is analyzed in an IROX 2000 FTIR fuel measurement device of Grabner Instruments. However, what must be mentioned, is that traces of C5 and C6 hydrocarbons are not condensed in the used condensation apparatus but are measured in the gas lump and accredited to the liquid lump. These traces cannot be analyzed with the IROX 2000 since they are not in the
Fig. 3. Particle size distribution of FCC catalyst. 3
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in Fig. 5. Contrary to the gas and gasoline lump the water content increases drastically with rising glycerol content. It rises from 11.1 wt% with pure canola oil to 59.9 wt% when using pure glycerol. Again, a linear regression was applied to describe the trend. The decrease in TFY and the increase in water content where expected due to the high oxygen content in the glycerol molecule and since in FCC, oxygen mainly converts to H2O. Regarding LCO + residue and coke lump the changes weren’t as severely as they were with other lumps. The LCO + residue lump decreases slightly with a stronger decline at higher glycerol contents. Contrary to that, rising glycerol content promotes the formation of coke with an increase from 6.8 wt% to 12.2 wt%.
Table 4 The Lump Model with analysis method. Fraction
Lump
Composition/ Boiling Range
Analysis method
Gaseous
Carbon oxide Gas Gasoline LCO Residue Water Coke
CO, CO2 C1 – C4 < 215 °C 215–320 °C > 320 °C
Infrared GC GC (Simdist) GC (Simdist) GC (Simdist) Gravimetric Paramagnetic, Infrared
Liquid
Solid
gasoline lump physically. The solid fraction, the coke that is deposited on the catalyst particles’ surface, is measured indirectly using the online measurement of the regenerator flue gas. In the regenerator the coke is combusted and the formed carbon oxides (CO, CO2) are detected with an infrared gas analyzer. The total fuel yield (TFY) describes the valuable products, which are the gas and gasoline lumps, in comparison to the feedrate.
TFY =
3.2. Composition of gas lumps In Fig. 6 the gas composition regarding valuable gases like olefins are depicted. The gas lump declines tremendously at higher admixtures of glycerol (see Fig. 4). Valuable products for polymer production like propylene are the main components of the gas lump when using pure canola oil. This does not change through the experimental runs. However, the feedbased amount decreases significantly from 12.3 wt% to 1.2 wt%. Naturally, since the gas lump itself decreases, each component has to decline as well with ethylene going down from 2.1 wt% to 1.1 wt % and butenes dropping from 8.4 wt% to 0.5 wt%. Regarding the composition of the gas lump different findings were discovered. The glycerol content has a drastic influence on the gas composition as well. Especially smaller gas molecules like propylene increase with rising glycerol content in the feed. As depicted in Fig. 7 the ethylene content of the gas rises from 7.4 wt% to 33.6 wt%. Propylene, however, decreases only when glycerol rich feeds are processed. Notably, between pure canola oil and 50 wt% glycerol in the feed no significant changes can be determined (propylene content around 43 wt %) whereas 35.2 wt% propylene are produced with pure glycerol. These results can be explained by the structure of glycerol since it comprises only 3 carbon atoms. Similar to propylene, butenes show the same trend with no significant changes until 50 wt% glycerol in the feed and a decrease with contents higher above that. Here, the decline is from around 30 wt% to 13.7 wt%. In addition to the gas lump the carbon oxide lump has been studied in detail as well. As depicted in Fig. 8, the combined carbon oxide lump as well as carbon monoxide and dioxide increase significantly when feeding higher admixtures of glycerol. CO rises from 2.0 wt% to 8.2 wt % and CO2 from 0.9 wt% to 3.2 wt%. Combined, the COx lump increases from 2.8 wt% to 11.4 wt%. The trend of the carbon oxides was described using a polynomic function of second order. The high amount
m˙ gas + m˙ gasoline m˙ feed
3. Experimental results The experiments were conducted at a constant feedrate of 2.7 kg/h and a riser temperature of 550 °C during all runs. The gained results for the product are depicted in the following figures. 3.1. Lumps and total fuel yield The values for the total fuel yield are depicted in Fig. 4. Here, it can be seen that a rising content of glycerol in the feed has a negative influence on the total fuel yield. With no glycerol in the feed the TFY is 68.5 wt%, while on the other hand, a feeding of pure glycerol leads to a TFY of 13.8 wt%. The values for glycerol-canola oil mixtures are in between. The influence of the glycerol content on the TFY can be described as approximately linear. Consequently, when the TFY decreases, the gas and gasoline lumps must decline as well. Here, a linear regression was also used to describe the trend. The gas lump decreased from 28.8 wt% to 3.3 wt%, while the gasoline lump went from 39.7 wt% to 10.5 wt%. The non-viable products LCO + residue, coke and water are shown
Fig. 4. TFY, gas and gasoline. 4
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Fig. 5. LCO + residue, coke and water.
Fig. 6. Composition of gas (feed based).
content is below 18 vol% in all the gasoline samples and therefore, fulfils the requirements [21]. The paraffin content, which is not directly limited through regulations, is between 31.4 wt% and 54.6 wt%. Like for carbon oxides a second order polynomic function was utilized to describe the trends. Information about the research octane number (RON) of the analyzed gasoline probes can be seen in Fig. 10. Here, the gasoline has a RON above 100 in all feed-mixtures. A minimal value of 95 is required by the applicable norms which is fulfilled [21]. Thus, the produced FCC gasoline is a suitable blending component to mix high RON gasoline.
of carbon oxide in the gas fraction derives from the high oxygen content in the glycerol molecule. 3.3. Composition of gasoline lump Beside the gas lump, the gasoline lump is the second economically valuable lump of the FCC products. It must be noted that sufficient liquid organic fraction could not be obtained when pure glycerol was used as feed. Therefore, no analysis of the gasoline lump was possible. In Fig. 9 the main components of gasoline are shown. It can be seen that the aromatics content of the gasoline first rises from 52.9 wt% to 63.0 wt% and then declines back to 39.4 wt%. It must be noted that none of the analyzed gasoline samples fulfil the requirement of maximum aromatics content which is limited to 35 vol% in DIN EN 228 [21]. Olefins, however, show a different trend. The highest olefin content is 11.2 wt% when pure canola oil is fed to the plant. As soon as glycerol is co-fed, the olefin content sinks. Unlike aromatics, the olefin
4. Comparison – glycerol/VGO – and glycerol/canola oil-mixtures As has been described by Swoboda et al. [22] several experimental runs of the TU Wien pilot plant utilizing vacuum gas oil (VGO)/glycerol mixtures have been conducted as well. Similar to the research done for this work, different feed mixtures were fed to the research plant ranging 5
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Fig. 7. Composition of gas (gas based).
gas and gasoline lump than canola oil feeds which can be explained by the significantly higher oxygen content in canola oil compared to VGO. Again the differences decrease at higher glycerol content in the feed mixtures. The water content, however, shows a different trend. The more glycerol is in the feed mixtures, the higher the water content rises. This applies to all experimental runs and, therefore, the interpretation that a serious amount of glycerol converts to water seems plausible. Notably, VGO itself does not convert to water whereas pure canola oil leads to 10 wt% in the product. The higher water content for canola oil mixtures is due to the catalytic conversions of the oxygen bonds in the canola molecule. The LCO + residue lump that is depicted in Fig. 13 shows a different trend described by a second order function whereas the previous mentioned lumps all show a linear trend. Again the differences between the 2 experimental series decrease at higher glycerol content in the feed and also the lumps in general decline. The higher amount of LCO + residue lump for the canola oil mixtures can be explained by the
from pure VGO (an industrial standard feed for FCC-plants) to pure glycerol. However, unlike glycerol and canola oil, which are miscible, a different feeding method using 2 different pumps was chosen since VGO and glycerol are not miscible. In this chapter, a comparison between canola oil/glycerol mixtures and VGO/glycerol mixtures regarding several key results was made to determine potential differences between these two experimental series. To determine if the co-feeding of glycerol is economically viable, the total fuel yields of the 2 experimental series were compared (Fig. 11). Here, it can be seen that the canola oil mixtures generally lead to a lower total fuel yield and, hence, to a smaller amount of valuable products. These differences between the 2 series decrease when feeding higher admixtures of glycerol. Consequently, the shown differences must primarily arise through the characteristics of canola oil and VGO. Both experimental runs show a linear trend. Since the TFY consists of the gas and gasoline lump the trends for both lumps are usually the same as for the total fuel yield (see Figs. 12 and 13). In general, using VGO mixtures as a FCC-feed lead to a higher
Fig. 8. Carbon oxides. 6
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Fig. 9. Paraffins, olefins and aromatics in gasoline lump.
Fig. 10. RON in gasoline lump.
Fig. 11. Comparison of TFY of canola oil mixtures vs. VGO mixtures.
glycerol content.
chemical structure. Unlike VGO which is a mix of different categories of molecules, canola oil is made up of long chained molecules (fatty acids) which are linked to a glycerol frame. Therefore, the LCO + residue lump which also consists of longer molecules is produced in a larger amount when using canola oil. Finally, a comparison between the propylene lumps was made since propylene is one of the most valuable products of the FCC-process. The feedbased content (Fig. 14) of the two mixtures shows a linear trend with decreasing amounts of propylene in the product. Here, VGO mixtures show a higher propylene content (feedbased) than canola oil mixtures. Interestingly, the gasbased content shows a different result. In mixtures with low glycerol content the canola oil mixtures lead to a higher gasbased propylene lump. This effect disappears at higher
5. Conclusion It could be shown that glycerol with a purity of 99.5 wt% and mixtures with canola oil can be processed in an FCC pilot plant. Experimental runs were conducted with pure canola oil and pure glycerol at a constant feedrate of 2.7 kg/h. Additionally, experiments with feed mixtures of 30 wt%, 50 wt% and 80 wt% glycerol were conducted. For all experiments the riser temperature was set to 550 °C. The findings are that under given conditions rising glycerol content in the feed leads to a decreasing conversion, meaning that the amount of valuable products like gases (C1-C4) and gasoline decreases which is 7
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Fig. 12. Comparison of gas and water lump of canola oil mixtures vs. VGO mixtures.
Fig. 13. Comparison of LCO + residue and gasoline content of canola oil mixtures vs. VGO mixtures.
Fig. 14. Comparison of feedbased and gasbased propylene lump between canola oil mixtures and VGO mixtures.
Put in numbers, it can be seen that glycerol mainly converts to water and carbon oxides with the water content surging from 11 wt% to almost 60 wt% and the carbon oxides increasing from 2.8 wt% to 11.4 wt %. An explanation can be found in the molecular structure of glycerol which is a trivalent alcohol. Thus, the three oxygen atoms in the molecule are the main cause for this behavior. Additionally, the reaction mechanism of catalytic cracking inherently promotes water formation if
undesirable. This trend can be described as linear leading to the lowest conversion rate when pure glycerol is used. The conversion rate plummets from 68 wt% to around 14 wt%. Corresponding to that, carbon oxide, water and coke lumps show a rising, linear trend. However, these lumps are of no commercial value which leads to the conclusion that from an economic point of view it is reasonable to only use feeds with a low glycerol content. 8
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References
oxygen is present in the feed. Since propylene and ethylene are the most valuable products of the FCC process the gas lump has been looked into in more detail. The main components of the gas lump namely ethylene, propylene, different butenes and other gases behave similar to the total gas lump. All show a linear decreasing trend with rising glycerol content in the feed. Especially the feedbased propylene content drastically decreases from around 12 wt% to a little over 1 wt%. This further indicates that cofeeding glycerol will only be viable if the glycerol content is low (around 10 wt% or less). Even though the results show that co-processing glycerol in the FCC unit is not promising regarding the product composition, the coke promoting properties of glycerol might still be useful. The FCC process runs mainly autothermal with the necessary energy required for the cracking reactions being provided by the burning of coke in the regenerator. VGO forms few coke during the process which can be a problem for process stability since too little energy is provided for the process. Therefore, often heavy residues are co-processed with VGO which promote coke formation. This role could also be taken over by glycerol since it also shows strong coke promoting properties. Thus, additional experimental runs with canola oil/glycerol mixtures at a low glycerol content are recommended to research its behavior in the process in more detail. The comparison between canola oil/glycerol mixtures and VGO/ glycerol mixtures has shown that in general the VGO/glycerol feed leads to a higher conversion and therefore a higher amount of valuable products. Hence, the addition of glycerol to a VGO feed is favorable in an economically point of view. Regarding the trends no essential difference was found between the two experimental series.
[1] Michael Anderl, et al., Klimaschutzbericht, Umweltbundesamt Österreich, Wien, 2016. [2] European Comission, "https://ec.europa.eu," 26 03 2019. [Online]. Available: http://appsso.eurostat.ec.europa.eu/nui/show.do?query=BOOKMARK_DS754546_QID_-730D1D9E_UID_-3F171EB0&layout=MOT_NRG,L,X,0;GEO,L,Y, 0;UNIT,L,Z,0;TIME,C,Z,1;INDICATORS,C,Z,2;&zSelection=DS-754546TIME, 2016;DS-754546INDICATORS,OBS_FLAG;DS-754546UNIT,NR;&rankNa. (Accessed 15 April 2019). [3] M. Tallet, et al., World Oil Outlook, OPEC, Vienna, 2016. [4] European Parliament, Directive 2009/28/EC of the European Parliament and of the Council, Brussels (2009). [5] Umweltbundesamt, Biokraftstoffe im Verkehrssektor, Umweltbundesamt Österreich, Wien, 2016. [6] R.L. Naylor, M.M. Higgins, The political economy of biodiesel in an era of low oil prices, Renew. Sustain. Energy Rev. (2017) 695–705. [7] J. Campelo, J.H. Clark, R. Luque, Handbook of Biofuels Production: Processes and Technologies, Woodhead Publishing, Philadelphia, 2011. [8] M. Ayoub, A.Z. Abdullah, Critical review on the current scenario and significance of crude glycerol resulting from biodiesel industry towards more sustainable renewable energy technology, Renew. Sustain. Energy Rev. 16 (2012) 2671–2686. [9] C. Quispe, C. Coronado, J.C. Jr, Glycerol: production, consumption, prices, characterization and new trends in combustion, Renew. Sustain. Energy Rev. 27 (2013) 475–493. [10] J.G. Speight, Petroleum Chemistry and Refining, Taylor & Francis, Washington DC, 1998. [11] R. Sadeghbeigi, Fluid Catalytic Cracking Handbook: An Expert Guide to the Practical Operation, Design, and Optimization of FCC Units, Elsevier Science, 2012. [12] M. Berchtold, J. Fimberger, A. Reichhold, Upgrading of heat carrier oil derived from liquid-phase pyrolysis via fluid catalytic cracking, Fuel Process. Technol. 142 (February) (2016) 92–99. [13] P. Bielansky, A. Weinert, C. Schönberger, A. Reichhold, Catalytic conversion of vegetable oils in a continuous FCC pilot plant, Fuel Process. Technol. 92 (December (12)) (2011) 2305–2311. [14] A. Weinert, P. Bielansky, A. Reichhold, Upgrading biodiesel into oxygen-free gasoline: new applications for the FCC-process, Apcbee Procedia 1 (2012) 147–152. [15] M. Wenchao, et al., Co-upgrading of raw bio-oil with kitchen waste oil through fluid catalytic cracking (FCC), Appl. Energy 1 (5) (2018) 233–240. [16] G. Huber, A. Corma, Synergies between bio- and oil refineries for the production of fuels from biomass, Angew. Chem. Int. Ed. Engl. 47 (2007) 7184–7201. [17] A. Primo, H. Garcia, Zeolites as catalysts in oil refining, Chem. Soc. Rev. 43 (2014) 7548–7561. [18] J. Melero, et al., "Production of biofuels via catalytic cracking,", Handbook of Biofuels Production, Woodhead Publishing Series in Energy, 2011, pp. 390–419. [19] A. Reichhold, Entwicklung von Reaktions/Regenerationssystemen für Adsorptions/ Desorptionsprozesse und für katalytisches Cracken auf der Basis von intern zirkulierenden Wirbelschichten, Technische Universität Wien, Wien, 1996 PhD thesis. [20] P. Bielansky, Alternative Feedstocks in Fluid Catalytic Cracking, Technische Universität Wien, Wien, 2012 PhD thesis. [21] DIN Deutsches Institut für Normung e.V, Kraftstoffe - Unverbleite Ottokraftstoffe Anforderungen und Prüfverfahren DIN EN 228, Beuth Verlag GmbH, Berlin, 2017. [22] M. Swoboda, M. Büchele, A. Reichhold, W. Hofer, Catalytic cracking of glycerol VGO mixtures in a continuously operated FCC pilot plant, manuscript in preparation, (2019).
Conflicts of interest The authors declare that they have no conflicts of interest regarding the publication of this article. Acknowledgments This work was supported by OMV Marketing and Refining GmbH (Österreichischer Mineralölverband). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cep.2019.107553.
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Canola oil/glycerol mixtures in a continously operated FCC pilot plant and comparison with vacuum gas oil/glycerol mixtures
T
⁎
Marco Büchelea, , Matthias Swobodaa, Alexander Reichholda, Wolfgang Hoferb a b
Vienna University of Technology, Institute for Chemical, Environmental & Bioscience Engineering (ICEBE), Getreidemarkt 9/166-3, 1060 Vienna, Austria OMV Marketing & Refining GmbH, Trabrennstraße 6-8, 1020 Wien, Austria
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluid catalytic cracking Glycerol VGO Canola oil Refinery technology
Catalytic cracking of glycerol/canola oil mixtures was conducted in a continuously operated FCC pilot plant at TU Wien. The range of mixtures was from pure canola oil to pure glycerol with several mixtures in between. The aim of this research was to prove if the addition of glycerol to the feed can be feasible and to determine changes in product quality. The set riser temperature was 550 °C and the selected feedrate was 2.7 kg/h. As bed material a zeolite-based commercially available FCC-equilibrium catalyst was used. Utilizing a 7 lump model the products were characterized. Viable products like cracking gas (C1–C4) and the gasoline lump (paraffins, olefins and aromatics) were analyzed in detail. The experiments showed that product distribution was mainly dependent on the glycerol content in the feed. The gas, gasoline and LCO + residue lump decreased dramatically. Non-viable products like water, coke and carbon oxides, on the other hand, increased significantly. Hence, it was concluded that under set conditions and parameters glycerol might only be beneficial to the FCC process in small admixtures because of possible control coke-make. Additionally, a comparison was made between experiments using canola oil/glycerol mixtures and VGO/glycerol mixtures.
1. Introduction Even though there are concerns regarding the exhaust fumes of diesel engines and their harmfulness to health, in particular NOx, diesel engine cars are still widely used in many parts of Europe. Especially in countries like Austria, Spain or France diesel engine cars are still a dominant factor regarding market share [1,2]. On a global perspective, the usage of diesel/gasoil is expected to at least stagnate on a high level with rising demand in developing countries and due to new regulations in the marine bunker sector [3]. Additionally, strict environmental regulations and the promotion of using higher quantities of environmental sustainable feedstock through legislation makes it challenging for refineries in the EU to satisfy the demand for middle distillates [4]. In Austria the widely used method of replacing fossil fuel is the addition of so called Biodiesel (FAME – fatty acid methyl ester). The addition of which is allowed up to the amount of 7 v% in Diesel so far [5]. Hence, the production of Biodiesel surged to fulfil the political target values. On a global perspective biodiesel production grew by around 23 percent annually between the years 2005 and 2015. This corresponds to a sevenfold increase in only a decade [6]. The main production process for biodiesel is the transesterification
⁎
of vegetable oils. There, a plant based oil like canola oil is esterified to FAME and the by-product glycerol using a base or acid as a catalyst [7]. Consequently, the leaping production of FAME leads to an exponential growth of glycerol production. This glycerol flooded the market and glycerol prices collapsed since demand remained stable [8]. Even price drops down to 10% of the original level were observed [9]. To resolve this issue, new applications for glycerol needed to be found. Therefore, the possible usage of glycerol as a cheap feedstock for fuel production was researched. One possible way is the co-processing of glycerol with other feedstocks in the fluid catalytic cracking (FCC) process which leads to this work. The FCC-process is one of the core processes in a refinery which basics were developed in the 1930s [10]. The basic principle behind the process is the cracking of low value high-boiling hydrocarbons into smaller molecules utilizing a catalyst and high temperatures. The valuable products produced are olefins (like ethylene and propylene), high-octane gasoline and to a smaller amount light cycle oil (LCO). Standard feeds for the catalytic cracking unit are normally gas oils from vacuum distillation units (VGO) but also heavy oil residues [11]. In general the FCC process is a very versatile and robust conversion process regarding feedstocks where different product spectrums can be obtained. This has been proven by various studies that utilized different
Corresponding author. E-mail address: [email protected] (M. Büchele).
https://doi.org/10.1016/j.cep.2019.107553 Received 8 February 2019; Received in revised form 7 May 2019; Accepted 30 May 2019 Available online 04 June 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
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feeds like e.g. vegetable oils or oils derived from pyrolysis [12–18]. Therefore, due to its flexibility and its establishment as one of the core refinery processes the FCC process is predestined to test alternative, more sustainable feedstocks. However, since industrial FCC plants produce around the clock and every production issue means money loss, feedstock tests on pilot plants are necessary to minimize economic risk for the industrial plant owner and prove viability. The objective of this work is to co-process glycerol/canola oil mixtures in an FCC pilot plant and to compare the results with another series of experimental runs of glycerol/vacuum gas oil mixtures conducted at the same pilot plant. At Vienna University of Technology (TU Wien) an FCC pilot plant has been developed by Hofbauer and Reichhold which was designed as an internally circulating fluidized bed system [19]. Later a second pilot plant was built by Bielansky and Reichhold which can process higher feedrates and introduced several improvements build in compared to the first plant [20]. Both plants show a high flexibility regarding process parameters (mainly riser temperature and feedrate) and can be operated with different feedstocks. Additionally, results of these pilot plants can be conveyed to industrial plants. This was shown in past cooperations with oil and gas companies when under similar setups (riser temperature and used catalyst) results of the pilot plant where compared with corresponding industrial FCC units.
Table 1 Key data. Pressure
Atmospheric
Regenerator temperature Riser temperature Total height Riser height Riser diameter Regenerator diameter Catalyst mass Feed rate Riser residence time Particle circulation rate C/O – ratio
500–800 °C 400–700 °C 3.2 m 2.5 m 0.0215 m 0.33 m 45 – 65 kg 2 – 8 kg/h ∼1 s 0.5 – 5 kg/min 10 – 50
Fig. 1. The corresponding key data is listed in Table 1. The plant is designed as an internally circulating fluidized bed system which results in a compact design. However, a side-effect of this construction type is the thermal coupling of the 2 plant sections: reactor and regenerator. The feed is pumped to a tubular oven using a gear pump where it is pre-heated. From there it flows through the feed inlet pipe into the riser where it evaporates immediately as soon as it gets in contact with the hot catalyst particles. This results in an increase in volume and thus the feed is transported pneumatically together with the catalyst particles to the top while cracking reactions occur. The residence time in the reactor is about 1 s. During the cracking reactions coke is generated which deposits on the catalyst particles and therefore, inactivates them. Once the catalyst-gas-mixture reaches the particle separator the spent catalyst is removed from the gas stream. The product gas leaves the plant at the top and is burned in a flare. A small portion of the product gas stream bypasses the torch and flows into a condensation apparatus where the products are gathered for analysis and separated into liquid and gaseous phase. The coke-loaded particles descend in a return flow pipe through the syphon into the regenerator section. Here, the particle bed is fluidized with air and coke deposits are combusted to provide the energy needed for the endothermal cracking reactions. The regenerated particles are then passing through the cooler system towards the bottom where they act again as a catalyst for the cracking reaction. This cycle provides a continuous plant operation. Except for the regenerator, all plant sections are fluidized with nitrogen which is especially important for the syphon and the bottom fluidization. Both act as a gas barrier that prevents oxygen from entering the product stream and product gas entering the regenerator. Additionally, the syphon acts as a stripper that removes remains of product gas from the spent particles. Parameters like regenerator temperature, riser temperature, particle circulation rate and feed rate can be adjusted in the ranges mentioned in Table 1. Depending on the chosen parameters the total residence time of feed in the plant is between 20–80 s.
2. Material and methods 2.1. Pilot plant A schematic of the FCC pilot plant located at TU Wien is shown in
2.2. Catalyst and feedstock The used FCC equilibrium catalyst is a zeolite – based catalyst developed for enhanced olefin and gasoline production. Its fluid dynamical properties were studied in advance to optimize plant operation. The minimal fluidization velocity was found to be 1.195 mm/s at 620 °C (Fig. 2). The catalysts particle size distribution is shown in Fig. 3. The mode, which defines the maximum of the particle size density distribution, is 82 μm. The feed utilized for the experiments consisted of glycerol and canola oil. In total 5 pilot plant runs were conducted where 3 different mixtures where fed into the plant with additionally one run with both pure glycerol and canola oil. Canola oil is a plant based oil which consists mainly of triglycerides with one glycerol molecule bound to 3 fatty acids. The hot filtrated canola oil was obtained from Rapso GmbH (Aschach, Austria). Some
Fig. 1. Pilot plant. 2
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Fig. 2. Pressure drop as a function of superficial velocity at 620 °C.
selected properties and the composition of fatty acids of canola oil are shown in Table 2. The glycerol utilized in this research is from the company Wasser Hygiene Chemie GmbH (Hilgertshausen-Tandern, Germany). It is solely plant-based and fulfils the quality requirements of the pharmaceutical and food industry. Some additional data is listed in Table 3.
Table 2 Fatty acid composition and physical and chemical properties of canola oil.
2.3. Analysis Due to the wide variety of hydrocarbons produced in cracking reactions an analysis model had to be developed that offers high accuracy with reasonable effort. This was solved using a so called lump model that is also used in industrial refining processes. Details of this 7 lump model are described in Table 4. The product gas is divided into a gaseous and liquid fraction utilizing a 3 stage condensation apparatus. The gained gaseous phase (not liquid at room temperature) is then analyzed in a gas chromatograph (SHIMADZU GC 17-A). Here, the gaseous fraction is studied using 2 separate columns and detectors. The gas lump (hydrocarbons C1 – C4) are analyzed using a FID and a Varian CP-Al2O3/Na2SO4 column. The N2 content is measured with a TCD and a CP CarboPLOT P7 column. The carbon oxide lump (CO, CO2) is measured online with a Rosemount NGA 2000 MLT3 of the company Emerson, an infrared gas analyzer. The liquid fraction consists of 2 phases, an aqueous and an organic one which are split up using a separation funnel. The aqueous phase that makes up the water lump is measured gravimetrically. The organic phase, on the other hand, is analyzed in a Simulated Distillation (SimDist) utilizing a second gas chromatograph. This GC is equipped with a Zebron ZB-1 column and a flame ionization detector (FID). Further information can be found in the supplementary data section. In a SimDist the organic phase is divided into 3 categories: gasoline, LCO and residue. Here, the separation criteria is the boiling range (Table 4).
Fatty acid composition Palmitic acid C16:0 Stearic acid C18:0 Oleic acid C18:1 Linoleic acid C18:2 Linolenic acid C18:3 Other
4.5 wt% 1.8 wt% 60.8 wt% 18.5 wt% 8.0 wt% 6.4 wt%
Physical and Chemical Properties Viscosity (40 °C) Density (20 °C) Bromine number Acid number Oxygen content Water content
35.9 mm²/s 915 kg/m³ 69.1 g Br/100 g 0.1 mg KOH / g 10.9 wt% < 0.1 wt%
Table 3 Glycerol properties. Density Purity Oxygen content Water content
1.26 kg/m³ > 99.5 wt% 52.1 wt% < 0.5 wt%
Additionally, a manual atmospheric distillation was conducted to gain a gasoline lump for further analysis. This gasoline lump is analyzed in an IROX 2000 FTIR fuel measurement device of Grabner Instruments. However, what must be mentioned, is that traces of C5 and C6 hydrocarbons are not condensed in the used condensation apparatus but are measured in the gas lump and accredited to the liquid lump. These traces cannot be analyzed with the IROX 2000 since they are not in the
Fig. 3. Particle size distribution of FCC catalyst. 3
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in Fig. 5. Contrary to the gas and gasoline lump the water content increases drastically with rising glycerol content. It rises from 11.1 wt% with pure canola oil to 59.9 wt% when using pure glycerol. Again, a linear regression was applied to describe the trend. The decrease in TFY and the increase in water content where expected due to the high oxygen content in the glycerol molecule and since in FCC, oxygen mainly converts to H2O. Regarding LCO + residue and coke lump the changes weren’t as severely as they were with other lumps. The LCO + residue lump decreases slightly with a stronger decline at higher glycerol contents. Contrary to that, rising glycerol content promotes the formation of coke with an increase from 6.8 wt% to 12.2 wt%.
Table 4 The Lump Model with analysis method. Fraction
Lump
Composition/ Boiling Range
Analysis method
Gaseous
Carbon oxide Gas Gasoline LCO Residue Water Coke
CO, CO2 C1 – C4 < 215 °C 215–320 °C > 320 °C
Infrared GC GC (Simdist) GC (Simdist) GC (Simdist) Gravimetric Paramagnetic, Infrared
Liquid
Solid
gasoline lump physically. The solid fraction, the coke that is deposited on the catalyst particles’ surface, is measured indirectly using the online measurement of the regenerator flue gas. In the regenerator the coke is combusted and the formed carbon oxides (CO, CO2) are detected with an infrared gas analyzer. The total fuel yield (TFY) describes the valuable products, which are the gas and gasoline lumps, in comparison to the feedrate.
TFY =
3.2. Composition of gas lumps In Fig. 6 the gas composition regarding valuable gases like olefins are depicted. The gas lump declines tremendously at higher admixtures of glycerol (see Fig. 4). Valuable products for polymer production like propylene are the main components of the gas lump when using pure canola oil. This does not change through the experimental runs. However, the feedbased amount decreases significantly from 12.3 wt% to 1.2 wt%. Naturally, since the gas lump itself decreases, each component has to decline as well with ethylene going down from 2.1 wt% to 1.1 wt % and butenes dropping from 8.4 wt% to 0.5 wt%. Regarding the composition of the gas lump different findings were discovered. The glycerol content has a drastic influence on the gas composition as well. Especially smaller gas molecules like propylene increase with rising glycerol content in the feed. As depicted in Fig. 7 the ethylene content of the gas rises from 7.4 wt% to 33.6 wt%. Propylene, however, decreases only when glycerol rich feeds are processed. Notably, between pure canola oil and 50 wt% glycerol in the feed no significant changes can be determined (propylene content around 43 wt %) whereas 35.2 wt% propylene are produced with pure glycerol. These results can be explained by the structure of glycerol since it comprises only 3 carbon atoms. Similar to propylene, butenes show the same trend with no significant changes until 50 wt% glycerol in the feed and a decrease with contents higher above that. Here, the decline is from around 30 wt% to 13.7 wt%. In addition to the gas lump the carbon oxide lump has been studied in detail as well. As depicted in Fig. 8, the combined carbon oxide lump as well as carbon monoxide and dioxide increase significantly when feeding higher admixtures of glycerol. CO rises from 2.0 wt% to 8.2 wt % and CO2 from 0.9 wt% to 3.2 wt%. Combined, the COx lump increases from 2.8 wt% to 11.4 wt%. The trend of the carbon oxides was described using a polynomic function of second order. The high amount
m˙ gas + m˙ gasoline m˙ feed
3. Experimental results The experiments were conducted at a constant feedrate of 2.7 kg/h and a riser temperature of 550 °C during all runs. The gained results for the product are depicted in the following figures. 3.1. Lumps and total fuel yield The values for the total fuel yield are depicted in Fig. 4. Here, it can be seen that a rising content of glycerol in the feed has a negative influence on the total fuel yield. With no glycerol in the feed the TFY is 68.5 wt%, while on the other hand, a feeding of pure glycerol leads to a TFY of 13.8 wt%. The values for glycerol-canola oil mixtures are in between. The influence of the glycerol content on the TFY can be described as approximately linear. Consequently, when the TFY decreases, the gas and gasoline lumps must decline as well. Here, a linear regression was also used to describe the trend. The gas lump decreased from 28.8 wt% to 3.3 wt%, while the gasoline lump went from 39.7 wt% to 10.5 wt%. The non-viable products LCO + residue, coke and water are shown
Fig. 4. TFY, gas and gasoline. 4
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Fig. 5. LCO + residue, coke and water.
Fig. 6. Composition of gas (feed based).
content is below 18 vol% in all the gasoline samples and therefore, fulfils the requirements [21]. The paraffin content, which is not directly limited through regulations, is between 31.4 wt% and 54.6 wt%. Like for carbon oxides a second order polynomic function was utilized to describe the trends. Information about the research octane number (RON) of the analyzed gasoline probes can be seen in Fig. 10. Here, the gasoline has a RON above 100 in all feed-mixtures. A minimal value of 95 is required by the applicable norms which is fulfilled [21]. Thus, the produced FCC gasoline is a suitable blending component to mix high RON gasoline.
of carbon oxide in the gas fraction derives from the high oxygen content in the glycerol molecule. 3.3. Composition of gasoline lump Beside the gas lump, the gasoline lump is the second economically valuable lump of the FCC products. It must be noted that sufficient liquid organic fraction could not be obtained when pure glycerol was used as feed. Therefore, no analysis of the gasoline lump was possible. In Fig. 9 the main components of gasoline are shown. It can be seen that the aromatics content of the gasoline first rises from 52.9 wt% to 63.0 wt% and then declines back to 39.4 wt%. It must be noted that none of the analyzed gasoline samples fulfil the requirement of maximum aromatics content which is limited to 35 vol% in DIN EN 228 [21]. Olefins, however, show a different trend. The highest olefin content is 11.2 wt% when pure canola oil is fed to the plant. As soon as glycerol is co-fed, the olefin content sinks. Unlike aromatics, the olefin
4. Comparison – glycerol/VGO – and glycerol/canola oil-mixtures As has been described by Swoboda et al. [22] several experimental runs of the TU Wien pilot plant utilizing vacuum gas oil (VGO)/glycerol mixtures have been conducted as well. Similar to the research done for this work, different feed mixtures were fed to the research plant ranging 5
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Fig. 7. Composition of gas (gas based).
gas and gasoline lump than canola oil feeds which can be explained by the significantly higher oxygen content in canola oil compared to VGO. Again the differences decrease at higher glycerol content in the feed mixtures. The water content, however, shows a different trend. The more glycerol is in the feed mixtures, the higher the water content rises. This applies to all experimental runs and, therefore, the interpretation that a serious amount of glycerol converts to water seems plausible. Notably, VGO itself does not convert to water whereas pure canola oil leads to 10 wt% in the product. The higher water content for canola oil mixtures is due to the catalytic conversions of the oxygen bonds in the canola molecule. The LCO + residue lump that is depicted in Fig. 13 shows a different trend described by a second order function whereas the previous mentioned lumps all show a linear trend. Again the differences between the 2 experimental series decrease at higher glycerol content in the feed and also the lumps in general decline. The higher amount of LCO + residue lump for the canola oil mixtures can be explained by the
from pure VGO (an industrial standard feed for FCC-plants) to pure glycerol. However, unlike glycerol and canola oil, which are miscible, a different feeding method using 2 different pumps was chosen since VGO and glycerol are not miscible. In this chapter, a comparison between canola oil/glycerol mixtures and VGO/glycerol mixtures regarding several key results was made to determine potential differences between these two experimental series. To determine if the co-feeding of glycerol is economically viable, the total fuel yields of the 2 experimental series were compared (Fig. 11). Here, it can be seen that the canola oil mixtures generally lead to a lower total fuel yield and, hence, to a smaller amount of valuable products. These differences between the 2 series decrease when feeding higher admixtures of glycerol. Consequently, the shown differences must primarily arise through the characteristics of canola oil and VGO. Both experimental runs show a linear trend. Since the TFY consists of the gas and gasoline lump the trends for both lumps are usually the same as for the total fuel yield (see Figs. 12 and 13). In general, using VGO mixtures as a FCC-feed lead to a higher
Fig. 8. Carbon oxides. 6
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Fig. 9. Paraffins, olefins and aromatics in gasoline lump.
Fig. 10. RON in gasoline lump.
Fig. 11. Comparison of TFY of canola oil mixtures vs. VGO mixtures.
glycerol content.
chemical structure. Unlike VGO which is a mix of different categories of molecules, canola oil is made up of long chained molecules (fatty acids) which are linked to a glycerol frame. Therefore, the LCO + residue lump which also consists of longer molecules is produced in a larger amount when using canola oil. Finally, a comparison between the propylene lumps was made since propylene is one of the most valuable products of the FCC-process. The feedbased content (Fig. 14) of the two mixtures shows a linear trend with decreasing amounts of propylene in the product. Here, VGO mixtures show a higher propylene content (feedbased) than canola oil mixtures. Interestingly, the gasbased content shows a different result. In mixtures with low glycerol content the canola oil mixtures lead to a higher gasbased propylene lump. This effect disappears at higher
5. Conclusion It could be shown that glycerol with a purity of 99.5 wt% and mixtures with canola oil can be processed in an FCC pilot plant. Experimental runs were conducted with pure canola oil and pure glycerol at a constant feedrate of 2.7 kg/h. Additionally, experiments with feed mixtures of 30 wt%, 50 wt% and 80 wt% glycerol were conducted. For all experiments the riser temperature was set to 550 °C. The findings are that under given conditions rising glycerol content in the feed leads to a decreasing conversion, meaning that the amount of valuable products like gases (C1-C4) and gasoline decreases which is 7
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Fig. 12. Comparison of gas and water lump of canola oil mixtures vs. VGO mixtures.
Fig. 13. Comparison of LCO + residue and gasoline content of canola oil mixtures vs. VGO mixtures.
Fig. 14. Comparison of feedbased and gasbased propylene lump between canola oil mixtures and VGO mixtures.
Put in numbers, it can be seen that glycerol mainly converts to water and carbon oxides with the water content surging from 11 wt% to almost 60 wt% and the carbon oxides increasing from 2.8 wt% to 11.4 wt %. An explanation can be found in the molecular structure of glycerol which is a trivalent alcohol. Thus, the three oxygen atoms in the molecule are the main cause for this behavior. Additionally, the reaction mechanism of catalytic cracking inherently promotes water formation if
undesirable. This trend can be described as linear leading to the lowest conversion rate when pure glycerol is used. The conversion rate plummets from 68 wt% to around 14 wt%. Corresponding to that, carbon oxide, water and coke lumps show a rising, linear trend. However, these lumps are of no commercial value which leads to the conclusion that from an economic point of view it is reasonable to only use feeds with a low glycerol content. 8
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References
oxygen is present in the feed. Since propylene and ethylene are the most valuable products of the FCC process the gas lump has been looked into in more detail. The main components of the gas lump namely ethylene, propylene, different butenes and other gases behave similar to the total gas lump. All show a linear decreasing trend with rising glycerol content in the feed. Especially the feedbased propylene content drastically decreases from around 12 wt% to a little over 1 wt%. This further indicates that cofeeding glycerol will only be viable if the glycerol content is low (around 10 wt% or less). Even though the results show that co-processing glycerol in the FCC unit is not promising regarding the product composition, the coke promoting properties of glycerol might still be useful. The FCC process runs mainly autothermal with the necessary energy required for the cracking reactions being provided by the burning of coke in the regenerator. VGO forms few coke during the process which can be a problem for process stability since too little energy is provided for the process. Therefore, often heavy residues are co-processed with VGO which promote coke formation. This role could also be taken over by glycerol since it also shows strong coke promoting properties. Thus, additional experimental runs with canola oil/glycerol mixtures at a low glycerol content are recommended to research its behavior in the process in more detail. The comparison between canola oil/glycerol mixtures and VGO/ glycerol mixtures has shown that in general the VGO/glycerol feed leads to a higher conversion and therefore a higher amount of valuable products. Hence, the addition of glycerol to a VGO feed is favorable in an economically point of view. Regarding the trends no essential difference was found between the two experimental series.
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Conflicts of interest The authors declare that they have no conflicts of interest regarding the publication of this article. Acknowledgments This work was supported by OMV Marketing and Refining GmbH (Österreichischer Mineralölverband). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cep.2019.107553.
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