Energy Fuels 2010, 24, 6103–6109 Published on Web 10/21/2010
: DOI:10.1021/ef101229b
Emerging Risks in the Biodiesel Production by Transesterification of Virgin and Renewable Oils E. Salzano,*,† M. Di Serio,‡ and E. Santacesaria‡ †
Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche (CNR), Via Diocleziano 328, 80124 Napoli, Italy, and ‡Dipartimento di Chimica, Universit a di Napoli “Federico II”, Via Cintia, 80126, Napoli, Italy Received June 22, 2010
Biodiesel is a very attractive biofuel because of its environmental benefits. However, despite its status as a safe substance, the production process can be hazardous because methanol or other flammable reactants, such as sodium methylate, can leave plants vulnerable to fire and explosion if not properly engineered and operated. However, further issues are emerging for the increasing capacity of plants, which have driven up on-site volumes of highly hazardous chemicals. Furthermore, some catastrophic accidents have occurred in very recent years in the glycerine neutralization phase. In this paper, insights of risks within this fairly new industry are presented. Heterogeneous catalysis seems to be the promising step for safer biodiesel productions. starting from 20 000 tons/year. Finally, most of the plants work with multiple feedstock, hence, both virgin and waste oils. In the framework of industrial safety, the scale-up of processes induced by the above-cited economical issues may produce a consistent increase of risks, because of the relevant amount of hazardous materials used in the process and the larger number and size of equipment. This alarming view is confirmed, at first glance, only by rapid analysis of the most relevant accidents in the biodiesel industry. According to the valuable work of Saraf,7 and with specific reference to plant operations, thus excluding transportation accidents following fire of biodiesel outside of the process site, in a 3 year period (2006-2009), there were eight fires and six explosions in the U.S. on the basis of about 200 biodiesel facilities, i.e., 2.5% annual probability of major accidents. These data are quite impressive if also considering that the total destruction of plants has occurred quite often. Eventually, biodiesel production plants should be considered as very risky processes, and the indulgent view of recent papers that claim exactly the opposite view should be contrasted.8,9 On the other hand, an in-depth analysis of accidents shows clearly that a simple analysis may overestimate the risks. Indeed, it can be shown that a relatively low dimension (and capacity of plants) is a common point for most of the examined incidents. To this regard, plants producing up to 35 000 tons/year are generally considered “backyard facilities”.10 These installations are typically start-up plants, and safety is likely to be compromised as entrepreneurs try to ~ez get into the industry at minimal costs. Also, Rivera and N un McLeod11 have studied the safety of discontinuous distillation
Introduction Biodiesel is defined as a fuel constituted of over 99% monoalkyl esters of long-chain fatty acids from vegetable oil or animal fat. Notwithstanding some criticism on its real environmental benefits,1 biodiesel is considered useful for the greenhouse gas (GHG) reduction, and its use is growing because of the law impositions,2 at least in the European Union (EU).3,4 Biodiesel is typically produced by transesterification of triglycerides of virgin oils or renewable sources (refined/edible oils), using methanol and alkaline catalysts. If virgin oil is used, the cost of feedstock lets the process be economically unfeasible with low-capacity plants. If waste oils are adopted, a lower cost of primary reagent is counterbalanced by the need of acid-catalyzed pretreatment processes (see the next section). As a consequence, because of a higher net annual profit and lower break-even price, the biodiesel production plants are considered remunerative (unless public contribution for waste oil treatment) if the annual capacity is increased to a minimum of 100 000 m3 of product/year.5 Figure 1 shows the plant capacity distribution and total biodiesel produced in the U.S. in 2010 (total capacity of 2 424 000 tons/year). Data are retrieved from Biodiesel Magazine.6 The plot shows that several small plants are active but that most of the production is concentrated in larger plants, *To whom correspondence should be addressed. E-mail: salzano@ irc.cnr.it. (1) Hoogeveen, J.; Faures, J.-M.; van de Giessen, N. Increased biofuel production in the coming decade: To what extent will it affect global freshwater resources? Irrig. Drain. Pap. 2009, 58, S148–S160. (2) European Parliament and the Council of the European Union. Directive 2009/28/EC. Off. J. Eur. Union, April 23, 2009. (3) European Biodiesel Board (EBB). 2009-2010: EU biodiesel industry restrained growth in challenging times. Press Release, July 22, 2010. (4) Marlair, G.; Rotureau, P.; Breulet, H.; Brohez, S. Booming development of biofuels for transport: Is fire safety of concern? Fire Mater. 2009, 33, 1–19. (5) You, Y.-D.; Shie, J.-L.; Chang, C.-Y.; Huang, S.-H.; Pai, C.-Y; Yu, Y.-H.; Chang, C. H. Economic cost analysis of biodiesel production: Case in soybean oil. Energy Fuels 2008, 22, 182–189. (6) Producing biodiesel plant list. Biodiesel Magazine 2010; http:// www.biodieselmagazine.com. r 2010 American Chemical Society
(7) Saraf, S. http://risk-safety.com, 2009. (8) Krawczyk, T. Biodiesel. INFORM 1996, 7, 801–822. (9) ASB Biodiesel (Hong Kong) Limited. Development of a biodiesel plant at Tseung Kwan O Industrial Estate. Environmental Impact Assessment Report, Executive Summary; Environmental Resources Management: Hong Kong, Oct 6, 2008. (10) McElroy A. K. Getting serious about safety. Biodiesel Magazine 2006, September. ~ez McLeod, J. E. Recommendations generated (11) Rivera, S. S.; N un about a discontinuous distillation plant of biofuel. Proceedings of the World Congress on Engineering; London, U.K., July 1-3, 2009.
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: DOI:10.1021/ef101229b
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Figure 1. Number and percentage of the total capacity distribution of producing a biodiesel production plant in the U.S. versus the plant capacity in 2010.
Figure 2. Transesterification reaction of triglycerides (TGAs) with methanol, catalyzed by alkalis (MþOH-), to fatty acid methyl esters (FAMEs, the biodiesel) and glycerol and competitive reactions with water [hydrolysis reaction of triacylglycerols (TAGs) to diacylglycerols (DAGs) and free fatty acids], following the free fatty acids with catalyst to soap and water. The R groups (R1, R2, or R3) are typically fatty acid chains (C12-C20). M is typically sodium or potassium.
plants of biodiesel by means of fuzzy failure mode and effect analysis (FMEA), arriving at the conclusion that, because of the simplicity of the discontinuous biodiesel process, systematic analyses are rarely applied and human intervention errors and lack of training in safety of the personnel are the main reasons for incidents. Besides these considerations, very few major incidents have been reported in Europe for the biodiesel industry (mainly in the U.K.). This evidence has to be clarified, even if the first hypothesis could be referred to the stricter European normative on licensing and safety procedures. This work deals with the new risks that are emerging in biodiesel production because of the impact of the recent economical crisis, which has addressed the producers to increase the year capacity of production and has induced the operators to use cheaper reactants for the purification phases of byproducts. These modifications are among the main reasons for several accidents that have occurred in the last few years. The analysis of the most relevant accidents in recent years is then presented. Attempts to clarify the discrepancy of accidental data between the U.S. and Europe are presented. Some insights are finally given on the risk related to the use of methanol and the post-neutralization procedures. Indeed, glycerine neutralization has been found as one of the most hazardous steps in the overall process. To this regard, the use of heterogeneous catalysis seems to be the cheapest and safest way for a biodiesel production plant because of the absence of the neutralization phase.
stirred-tank reactor or in a continuous process as the plug flow reactor or combined stirred-tank reactor (STR) at 400 kPa.15,16 Sodium or potassium hydroxide or related methoxides (potassium methanolate, CH3OK) are typically adopted as catalysts. A typical molar ratio of methanol/triglycerides in the alkaline-based process is 6:1.17 This ratio needs to be higher than stoichiometric to drive the equilibrium to maximum ester yields. Indeed, with a large methanol excess, the intermediate species (di- and monoglycerides) are only present at a very early stage of the reaction. The products exiting from the main reactor are in two liquid phases, which are separated by a settling tank often followed by centrifugation. The two streams, the glycerol phase and the fatty acid methyl esters (FAMEs), which constitute the main final product of biodiesel, are both separated from methanol in distillation towers with few stages (typically five) because of the large differences in the boiling point of the components. Because of FAME and glycerol thermal instability, vacuum distillation at low temperature (below 150 °C) conditions are adopted in this stage. Often, before the methanol distillation, FAME- and glycerolbased streams are neutralized with mineral acids. The methanol stream is then purified by vacuum distillation to remove water and other contaminants before its recycling to the reactors. To achieve the stated technical international standard (EN 14214 or ASTM D 6751), the FAME stream is then purified with water in a liquid-liquid extraction washing column. The purified FAME stream is further dried by water vacuum distillation. After neutralization, crude glycerol with 85% purity, with 15% water and salts derived from catalyst neutralization, is obtained. The stream is typically purified by vacuum distillation. Many low-cost feedstock oils (e.g., waste cooking oil) are available for biodiesel production. Unfortunately, those oils often contain large amounts of free fatty acids (FFAs), which react with alkaline catalysts to produce soaps (e.g., potassium or sodium oleate) and water (see Figure 2). Soaps of saturated fatty acids tend to solidify at ambient temperature, thus forming an
Biodiesel Production Process Transesterification of triglycerides of virgin oils or refined/ edible oils using methanol and alkaline catalysts is nowadays largely adopted for biodiesel production12-14 (Figure 2). The alkaline-catalyzed reaction is performed at about 60 °C (near the boiling temperature of methanol) and ambient pressure in a (12) Ma, F.; Hanna, M. A. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1–15. (13) Pinto, A. C.; Guarieiro, L. L. N.; Rezende, M. J. C.; Ribeiro, N. M.; Torres, E. A.; Lopes, W. A.; de Pereira, P. A.; de Andrade, J. B. Biodiesel: An overview. J. Braz. Chem. Soc. 2005, 16, 1313–1330. (14) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakaran, K.; Bruce, D. A.; Goodwin, J. G., Jr. Synthesis of biodiesel via acid catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353–5363. (15) Van Gerpen, J. Biodiesel processing and production. Fuel Process. Technol. 2005, 86, 1097–1107.
(16) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour. Technol. 2003, 89, 1–16. (17) Freedman, B.; Pryde, E. H.; Mounts, T. L. Variables affecting the yields of fatty esters from transesterified vegetable oils. J. Am. Oil Chem. Soc. 1984, 61, 1638–1643.
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Table 1. List of Relevant Accidents Occurring in 2006-2007 in Biodiesel Production Plants number
date
company
1
02/17/2006
American Biofuels, Bakersfield, CA
2
06/23/2006
Sunbreak Biofuels, Canby, OR
3
07/07/2006
Blue Sky Biodiesel, New Plymouth, ID
4
07/17/2006
Diester Industrie, Venette, France
5
05/27/2007
Oleon NV, Ertvelde, Belgium
6
07/14/2007
Agri Biofuels, Dayton, TX
7
07/25/2007
Better Biodiesel, Spanish Fork, UT
8
08/21/2007
9
08/25/2007
Farmers and Truckers Biodiesel, Augusta, GA Foothills Biodiesel, Lenoir, NC
10
09/02/2007
San Diego, CA
description during the transfer of methanol, the transporting tote slipped from its carrier, leading to the outbreak of the fire; the methanol was ignited possibly because of static electricity, and the fire spread into the process building; the entire plant was destroyed as a result of the fire; multiple explosions were also recorded a major fire resulted starting from a small fire of a plastic biodiesel storage tank; storing biodiesel in plastic tanks is not good manufacturing practice;24 the incident occurred in a startup biodiesel company a worker at the plant was welding on top of a large indoor tank containing soy oil when there was an explosion at that tank; both the explosion and fire destroyed the biodiesel plant; the incident is directly linked to well-known errors on maintenance activities of tanks43 a chimney fire resulting from malfunctioning of a process safety device at the biodiesel production unit, however, with very limited damages4 a fire occurred in the 190 000 tons/year fatty acid plant; because of a leak in the distillation column, fat at a high temperature came in contact with oxygen; nobody was injured; however, the material damage to the installations was important a fire resulted from a methanol spill; the facility was evacuated; no injuries were reported a small fire occurred when there was a mechanical malfunction in a methanol transfer line to the reactor section a worker was welding a flow meter on the top of a tank and was killed when an explosion occurred feedstock tanks in the tank farm were destroyed by fire 2 days after the plant was shutdown; no one was hurt a fire at a biodiesel plant left one worker severely burned
processes) over about 100 incidents. Different clusters of accidents were then recognized; two of them refer specifically to the process plant (however, excluding backyard installation) and transportation of hazardous materials, respectively. Results have shown that about 20% of biofuel incidents are related to the explosion or fire in the storage tank area. About 25% of the overall number of incidents regard the transportation of hazardous substances. Within process plant incidents, more than 47% have occurred during maintenance operations. These data, although related to biofuels only, confirm the previous data by Duguid. Also, recently but more specifically related to the alkalinebased biodiesel process, ASB Biodiesel Ltd. has published the results of the risk analysis within the public enquiry for the construction of a new biodiesel production plant in the Hong Kong area.9 The risk analysis includes the incidents that occurred during transportation and maintenance operation, for all hazardous materials stored or produced in the plant. The incidents list analyzed by ASB derive from accident databases (either commercial or freely available on the web) typically adopted in risk assessment. However, it missed some cases in the recent years, although catastrophic, probably because they were too recent for the inclusion in the same databases or because specific data were sometimes scarce. Finally, the valuable work of Saraf7 has constituted the starting basis for the historical analysis reported in this work. In the following Tables 1-3, the most relevant case histories collected in the last 4 years (2006-2009) as published in the literature,4,21,22 historical databases (as retrieved from ABS analysis9), and newspaper and specialized internet reports (e.g., refs 23-25) are shown. No accidents because of
undesired gel and semi-solid mass. Furthermore, the water formed in the previous described reaction can hydrolyze the triglycerides, thus forming a new fatty acid and diglycerides. Eventually, a pretreatment process at 70 °C and 400 kPa, with methanol and sulfuric acid, is generally adopted to reduce to negligible values the amount of FFAs before the transesterification reaction. This pretreatment phase includes a methanol recovery by vacuum distillation to remove water before alkaline transesterification. Furthermore, the necessity of sulfuric acid neutralization before the transesterification stage leads to a larger use of basic catalyst and salt production. To solve this problem, a solid (heterogeneous) acid catalyst has also been proposed.18 To this regard, sulfonic resins have shown very good results for what concern catalyst activity, catalyst life, and its regenerations,19 with specific reference to fatty acid esterification.
Relevant Biodiesel Incidental Cases in the Last 5 Years Ruling out transportation accidents, Duguid20 has stated that most incidents occur during idle or shutdown (15%), startup (14%), maintenance (11%), and abnormal (13%) operations; e.g., 50% of the total number of incidents does not involve the process under normal conditions. Furthermore, of the remaining half, 22% of incidents are related to tank storage (e.g., overfilling, leakages, and tank fires). These data can be compared to the data produced recently by Rivier and Marlair21 for the entire set of biofuel produc tion (e.g., considering both ethanol- and methanol-based (18) Jeromin, L.; Peukert, E.; Wollomann, G. U.S. Patent 4,698,186, 1987. (19) Tesser, R.; Casale, L.; Verde, D.; Di Serio, M.; Santacesaria, E. Kinetics and modeling of fatty acids esterification on acid exchange resins. Chem. Eng. J. 2010, 157, 539–550. (20) Duguid, I. M. Analysis of past incidents in the oil, chemical and petrochemical industries. Loss Prevention Bulletin; Institution of Chemical Engineers: Rugby, U.K., 1998; Vol. 142, pp 3-6. (21) Rivier, C.; Marlair, G. The use of multiple correspondence analysis and hierarchical clustering to identify incident typologies pertaining to the biodiesel industry. Biofuels, Bioprod. Biorefin. 2010, 4, 53–65.
(22) Shird, M. M.; Williams, M. R. Oak biodiesel: State buildings and fire codes affecting biodiesel manufacturing. Proceedings of the 2007 National Conference on Environmental Science and Technology; Greensboro, NC, Sept 12-14, 2007; pp 91-96. (23) Moss, K. White Paper Biodiesel Plant Safety; Frazier, Barnes and Associates, LLC: Memphis, TN, 2009; December. (24) Moss, K. Biodiesel plant safety. Biodiesel Magazine 2010, March. (25) Industrial Fire World (IFW). http://www.fireworld.com, 2010.
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Table 2. List of Relevant Accidents Occurring in 2008 in Biodiesel Production Plants number
date
company
11
01/03/2008
American AG Fuels, Defiance, OH
12
04/15/2008
Western Biodiesel, Calgary, Alberta, Canada
13
05/28/2008
Greenlight Biofuels, Princess Anne, MD
14 15 16 17
06/28/2008 07/01/2008 08/15/2008 08/27/2008
Serembain, Malaysia Winchester, KY Davis, TN All American Biodiesel, York, ND
18
08/15/2008
Biofuels of Tennesse, Decaturville, TN
19
09/18/2008
Gadsten Fleet Management Facilities, Gadsten, AL
20
09/30/2008
Nova Biosource Fuels, Clinton, IA
21
11/22/2008
Antakya, Turkey
number
date
22
02/09/2009
GreenHunter Biofuels, Houston, TX
23 24 25
02/20/2009 03/21/2009 05/24/2009
26 27
06/04/2009 06/14/2009
Kountze, TX Athens, AL Minnesota Soy Bean Processors, Brewster, MN Burnley, U.K. Midwest Biorenewables, Toledo, OH
28
07/04/2009
29 30
07/16/2009 08/19/2009
Gen-X Energy Group, Inc., Burbank, WA Duzco, Turkey Columbus Foods, Chicago, IL
31 32
08/19/2009 09/23/2009
Cuiaba, Brazil New Eden Energy, St. Cloud, FL
33 34
10/14/2009 12/02/2009
Xenerga Biodiesel, Savannah, GA Imperium Renewables, Hoquiam, WA
35
12/02/2009
Aberdeen, WA
description an explosion related to the accumulation of methanol vapors inside a glycerine tank; the vapors were released through an open manhole, forming a cloud that was ignited by a motorized door; the plant produced about 20 000 tons of biodiesel/year an explosion at a biodiesel plant because of welding operation on top of the biodiesel-settling tank; fumes and methanol accumulated inside the tank ignited by the welding operation caused the explosion; fire continued to burn in the tank for several hours an explosion in a biodiesel plant killed one and injured another worker while welding pipes at the idled plant a small fire broke out at a biodiesel plant a small fire broke out at a biodiesel plant a fire destroyed a biodiesel plant one of four processing buildings and the processing equipment were destroyed by fire; destruction of the equipment was so great that it was difficult for investigators to ascertain an explosion took place in a standby biodiesel plant awaiting conversion to glycerine production; the explosion and fire destroyed all of the existing stocks of biodiesel, sodium hydroxide, methanol, and glycerine; no injuries were reported; the plant produced about 50 000 tons of biodiesel/year a faulty heating element on the biodiesel equipment caused the top of the tank to blow off because of an explosion of vapors; equipment had a burst plate, which was designed to go off and relieve pressure rather than the tank exploding when pressure builds up; the hole in the roof was about 1 ft in diameter a small fire in the primary biodiesel recovery column was quickly extinguished by the local fire department; the probable cause was a buildup of methanol vapors in the column during a ventilation process required as part of the maintenance activity; no injuries were reported for plant personnel; one fireman received minor steam burns three workers were killed and five were injured in a fire at a biodiesel plant
Table 3. List of Relevant Accidents Occurring in 2009 in Biodiesel Production Plants company
description a mechanical seal on a circulation pump associated with a process heating unit failed; the resulting excessive heat created caused the fire a fire damaged a biodiesel facility a fire at a biodiesel plant left a worker in critical condition a fire and explosion resulted in several tanks being on fire a fire broke out at a biodiesel plant a vacuum-control valve on a vacuum-refining vessel failed to work, causing implosion; no injuries were reported; the plant produced about 1000 tons of biodiesel/year a large amount of vegetable oil spilled when a fire broke out at a biodieselmanufacturing plant an explosion rocked a biodiesel plant an explosion causing partial collapse of the main building and shocking nearby buildings occurred in the biodiesel plant; a plume of smoke smelling strongly of chemicals and sulfur lingered in the air for several minutes afterward; the plant produced about 10 000 tons of biodiesel/year; the explosion was set off in a mixing tank when a worker improperly mixed glycerine and sulfuric acid an explosion at a biodiesel plant injured three people a warehouse containing tons of methanol and biodiesel exploded and was on fire; the explosion was possibly triggered by the rupture of the sulfuric acid tank one injury resulted from an explosion in a reactor used to store biodiesel a tank of about 50 m3 containing glycerine exploded as a result of overpressurization during neutralization with sulfuric acid; two adjacent tanks containing sulfuric acid were also damaged after the explosion, but no injuries occurred; the Imperium Renewables plant in Hoquiam is one of the largest biodiesel plants in the U.S. (340 000 ton/year capacity) sulfuric acid leaked into a containment area after an explosion at a biodiesel plant
transportation are reported if they occurred outside the process plant, as in the case of road or rail accidents. Worker accidents in a garage-type plant because of bad practice or total inexperience in biodiesel production are also not reported, because they are not considered specific for the analysis reported here. For this type of accident and the transportation accidents, the causes and evolution of
the accident and the prevention and mitigation procedures are clearly very similar for any process and will not be discussed. Results and Discussion From the data and the process description reported in the previous section, it is clear that biodiesel has to be considered 6106
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Table 4. Equipment Size for the Biodiesel Industry by Varying the Production Capacity9,16 a equipment
size
transesterification reactor D L
8000 tons/year
Table 5. Failure Frequencies in the Biodiesel Process Area as Used by ASB9
100 000 tons/year
1.8 5.4 (1)
2.7 8.1 (3 parallel) neutralization reactor DL 0.3 1.0 (1) 0.7 2.3 (1) washing column DH 0.8 10.0 (1) 0.8 10 (12 parallel) FAME distillation tower D H 1.2 12.0 (1) 1.2 12 (12 parallel) 500 methanol buffer tank volume (m3) 5 10 (2/day) loading of methanol volume (m3)
vessels
leak frequency (year-1)
rupture frequency (year-1)
esterification vessel methanol buffer tank pipe reactor, 350 m methanol recycle tank
2.2 10-4 1.1 10-4 1.75 10-4 1.1 10-4
2.0 10-5 1.0 10-5 3.5 10-5 1.0 10-5
Starting from the data in Tables 4 and 5, ASB has presented the overall risk analysis, thus reporting the potential loss of life (PLL) and societal risk results. Details on the significance of PLL as societal risk quantification are reported elsewhere.29,30 For the sake of clarity, only definitions are reported here. Societal risk may be defined as the relationship between the frequency, F, and the number, N, of people suffering from a specified level of harm in a given population from the realization of specified hazards. In an industrial environment, this relationship is often represented graphically by the F/N curves, which displays the year-based probability of exceedance of the number of fatalities because of any credible industrial accident that may occur in the analyzed area. The expected value of the number of fatalities per year, E(N), which is the area (integral) of the F/N curve is called the PLL. For the ASB plant, the calculated value for the PLL is 6.83 10-7 year-1, which represents a very low risk value if compared to the typical threshold value for acceptability as given, e.g., by Vrijling et al.31 The study also evaluates the relative contribution of each scenario over the entire set of analyzed accidents, which may then be generalized. More than half (55%) of PLL per year is given by methanol leak or rupture from pipelines, thus resulting in the explosion of vapors [vapor cloud explosion (VCE)]. The total contribution of methanol increases to 70% if adding the risks related to the transesterification reactor, the methanol tank, and the neutralization tank. With respect to the F/N curves, the risks related to the process are mainly characterized by the total number of fatalities (N) between 3 and 10; i.e., only small consequences on workers or the population are expected. For greater values of N, the tank farm accidents are the only contribution but very low frequencies are expected. In our opinion, the ASB analysis should be re-evaluated, taking into account recent accidents; i.e., the assessment is mainly concentrated on methanol, but other substances should be considered. On the other hand, accidents related to methanol transportation should be restricted to the plant border only, e.g., during transfer line accidents, thus neglecting the accidental scenarios related to rail-tank or road-tanker accidents. With respect to the methanol use in biodiesel production, inherent safety principles have also been applied to biodiesel production by Mannam et al.,32 following the well-known guidewords: intensification, substitution, attenuation, and
a
D, L, and H are diameter, length, and height, respectively, in meters. Data in parentheses show the number of equipment.
as a very safe fuel, because of its intrinsic safety physicochemical characteristics. Hence, only the domino effect, e.g., during fires, should be analyzed. As a consequence, risk analysis in the biodiesel production process should be related, at least in a first attempt, to the hazards of methanol (and methoxides) only, in all of the process phases from the transesterification reactor to the distillation towers and, of course, to storage tanks. However, some unexpected accidents have occurred in neutralization phases for glycerine. These aspects will be discussed in the next sections. Risks of the Process with Respect to Methanol. The risks related to methanol use are well-known in the literature, even with specific reference to the biodiesel production process,26-28 and will not be discussed for the sake of brevity. However, some new risks are related to the fast increase of plant annual capacity, which has strongly enhanced the hazards of biodiesel production and the consequences of accidental scenarios, because of the increased complexity of plants, the number and dimension of equipment, and the larger inventory of chemicals. To this regard, the modification in plant characteristics when increasing the plant capacity may be observed if comparing the data of Zhang et al.15 for an 8000 tons/year production plant (a “backyard production”) to the data of ASB9 (Table 4). The design and process data in Table 4 can be combined with failure frequencies of equipment as given by the same ASB analysis (Table 5). Quite clearly, the risk values are strongly affected by the largest complexity of 100 000 tons/year plants. Also, larger plants include a buffer tank (500 m3 of methanol, working time of about 2 weeks for the ASB plant) and two road tankers (10 m3) per day, thus increasing the critical aspect of loading/unloading risks, even if, as for the specific case of ABS, one barge of 1000 tons/week is normally received by the nearby harbor and road tankers are used only in the case of impossibility of sea transport of methanol. On the other hand, increasing complexity also means more advertised safety management. As in Europe, few small backyard plants are licensed; this point could be the first response to the disproportionality between European and U.S. accidents.
(29) Jonkman, S. N.; van Gelder, P. H. A. J. M.; Vrijling, J. K. An overview of quantitative risk measures for loss of life and economic damage. J. Hazard. Mater. 2003, A99, 1–30. (30) Institution of Chemical Engineers. Nomenclature for Hazard and Risk Assessment in the Process Industries; Institution of Chemical Engineers: Rugby, U.K., 1985. (31) Vrijling, J. K.; van Hengel, W.; Houben, R. J. Acceptable risk as a basis for design. Reliab. Eng. Syst. Saf. 1988, 59, 141–150. (32) Mannan, M. S.; Wang, Y.; Zhang, C.; West, H. H. Application of inherently safer design principles in bidiesel production process. Proceedings of Hazard XIX; Institute of Chemical Engineers, Manchester, U.K., 2006; pp 982-989.
(26) World Health Organization (WHO). Environmental Health Criteria 196: Methanol; WHO: Geneva, Switzerland, 1997. (27) Kemper, T. Biodiesel plant;Safety overview. National Biodiesel Conference; Orlando, FL, 2008. (28) Perry, J. A. Catastrophic incident prevention and proactive risk management in the new biofuels industry. Environ. Prog. Sustainable Energy 2009, 28, 72–82.
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Table 6. Compatibility Matrix for Glycerol Byproducts as Given by NOOA, Chemical Reactivity Worksheet33 a H2SO4 H2SO4 KOH glycerol CH3OH H2O
A6, B1, C, D6, D7 A9, B1, B6, C A9, B1, B6, C D6, D7
KOH
glycerol
CH3OH
H2O
A6, B1, C, D6, D7
A9, B1, B6, C B1, B5, C
A9, B1, B6, C B1, B5, C
D6, D7 C, D3, D6, D6, D7 B5, D7, E
B1, B5, C B1, B5, C C, D3, D6, D6, D7
B5, D7, E
a
A6, reaction proceeds with explosive violence and/or forms explosive products; A9, heat generation from the chemical reaction may initiate explosion; B1, may become highly flammable or may initiate a fire, especially if other combustible materials are present; B4, spontaneous ignition of reactants or products because of reaction heat; B5, combination liberates gaseous products, at least one of which is flammable, and may cause pressurization; B6, combination liberates gaseous products, including both flammable and toxic gases, and may cause pressurization; C, exothermic reaction that may generate heat and/or cause pressurization; D3, combination liberates gaseous products, at least one of which is toxic, and may cause pressurization; D5, combination liberates combustion-enhancing gas (e.g., oxygen) and may cause pressurization; D6, exothermic reaction and generation of toxic and corrosive fumes; D7, generation of corrosive liquid; E, generation of water-soluble toxic products.
limitation, as strategies to remove or reduce hazards. Quite clearly, reducing the amount of methanol is essential within an inherent safety strategy. To this aim, the work of Mannam et al. has been addressed to the use of a new continuous oscillatory flow reactor, to improve mixing (i.e., under the process intensification keyword because of the enhanced heat- and mass-transfer limitation). This option only partially decreases the hazardous condition of the process because of the slightly reduced methanol ratio and does not solve many of the issues related to the plant capacity. The use of other alcohols (substitution) is currently under development, even if the process conditions are different and other problems arise. Risk of the Glycerine Neutralization Phase. The price of sulfuric and phosphoric acids may vary strongly, following worldwide oscillation of markets. However, sulfuric acid costs only a fraction of the more expensive phosphoric acid, even including the revenues of sodium or potassium phosphate salts deriving from the neutralization of the alkaline catalyst. This point has addressed, quite easily, the producers on the cheaper alternatives, but the use of concentrated sulfuric acid as a neutralization agent, also in a large-scale plant, has introduced new hazards that should be analyzed. Thus far, the starting point for this new evaluation can be the compatibility table, as given by the code of National Oceanic and Atmospheric Administration (NOAA),33 for the typical glycerol byproduct obtained from alkalinecatalyzed transesterification (Table 6), as also suggested by Saraf.7 Here, it is also worth mentioning that phosphoric acid is only considered as “C” in the compatibility chart when mixing with either glycerol or methanol. From Table 6, the risks because of mixing of sulfuric acid and glycerol byproduct are easily observed, and this operation can lead to explosion, equipment overpressurization, and fires. That is, the biodiesel accidents that have recently occurred in the neutralization operation, which is generally perceived at very low risk, can simply be related to the low knowledge of risks for the management and operators involved in that operation when using sulfuric acid. Just to say, in the case history, the overpressurization of the reactor was declared by the management as the result of an oversupply of sulfuric acid into the glycerine neutralization tank, which caused an “unexpected” exothermic reaction. It should also be noted that, because of low knowledge, the processing equipment in that case was not designed with physical or mechanical safeguards to prevent an oversupply
of sulfuric acid, that the same company has now replaced the glycerine neutralization tank with a completely new system, equipped with stringent safeguards to prevent an oversupply of sulfuric acid, and that new training and safety programs onsite have been provided. With specific reference to the biodiesel production, Banavali et al.34 have reported that if crude glycerol (as a biodiesel byproduct after the transesterification reaction) is mixed with a stochiometric amount of 96% sulfuric acid in a vessel with a stirrer operating at 400 rpm, the temperature of the mixture may rise from room temperature to 65 °C. This increase could even be higher if a great excess of sulfuric acid is used. Moreover, if the mixing is not sufficient, higher local temperature increases are possible. The increase of the temperature may then lead to dangerous conditions in the process equipment, and pressurization of the tank may occur. To the authors’ knowledge, two routes may be thought for the explosion of the neutralization tank because of pressurization, which have to be clarified. The first route could be related to the formation of an explosive gaseous phase because the evaporation of residual methanol in glycerol. This evaporation is due to the increase of the temperature, which occurs during the sulfuric acid feeding. The second route is the consequence of unefficient mixing of concentrated sulfuric acid and is related to the formation of propenale (acrolein), following the classical dehydration reaction from glycerol35-37 (Figure 3) at high temperatures. The reaction needs heating of liquid to 170 °C and is catalyzed by sulfates. Sulfuric acid may also catalyze the reaction because it decreases the temperature of decomposition of acrolein to polyglycerols, chiefly diglycerol C6H1405.38 Acrolein is strongly toxic, may cause severe irritation to operators, and is characterized by a typical smell of smoke, sometimes reported by accident witnesses. The gaseous acrolein may pressurize the neutralization reactor together with vaporized methanol as a result of heating. (34) Banavali, R. M.; Hanlon, R. T.; Schultz, A. K. Method for purification of glycerol from biodiesel production. U.S. Patent 7,534,923, 2008. (35) Adkins, H.; Hartung, W. H. Acrolein. Org. Synth. 1941, 1, 15. (36) Castell o, M. L.; Dweck, J.; Aranda, D. A. G. Thermal stability and water content determination of glycerol by thermogravimetry. J. Therm. Anal. Calorim. 2009, 97, 627–630. (37) Nimlos, M. R.; Blanksby, S. J.; Qian, X.; Himmel, M. E.; Johnson, D. K. Mechanisms of glycerol dehydration. J. Phys. Chem. A 2006, 110, 6145–6156. (38) Watanabe, M.; Iide, T.; Aizawa, Y.; Aida, T. M.; Inomata, H. Acroleyn synthesis from glycerol in hot-compressed water. Bioresour. Technol. 2007, 98, 1285–1290.
(33) Office of Response and Restoration, National Ocean Service, National Oceanic and Atmospheric Administration (NOAA). Chemical Reactivity Worksheet; NOAA: Washington, D.C., 2010.
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could be the main reason for the relatively low number of large-scale accidents in Europe, where backyard plants are rare with respect to U.S. plants. Economical reasons have addressed to the use of sulfuric acid in the neutralization process of glycerol byproducts, thus introducing new risks, because of the incompatibility between mineral acid and glycerol, which may pressurize the reactor when excess acid is used. From what was said above, several operations involving large use of chemicals and large consumption of energy are necessary for the purification of transesterification products. These problems are mainly addressed by the use of homogeneous catalysts, which are not easy to remove from the reaction products. Increasing the control over the neutralization process is essential, but the use of heterogeneous catalysts may solve the problem inherently. To this regard, several processes have been reported in the recent literature, but until now, only the process of the Institute Franc-ais du Petrole (IFP Energies Nouvelles) is operative.41 The IFP technology operates at 200-250 °C but does not require catalyst neutralization and aqueous biodiesel treatment steps. Hence, glycerol is directly produced with no neutralization step, with high purity levels and free from any salt contaminants. A more in-depth analysis is however necessary to compare the safety of heterogeneous processes with the classical homogeneous catalyst process. As a matter of fact, even if in this case the purification section of the plant is really simplified (thus, avoiding the necessity of the neutralization step), the process is more hazardous because of higher pressures (40-70 bar) and temperatures (200-220 °C) adopted. Furthermore, a higher methanol excess (from 1:10 to 1:30) is used, as compared to the homogeneous catalysis process. However, it is worth pointing out that great efforts are being performed worldwide to find new heterogeneous catalysts that can operate at lower temperatures (60-100 °C).42 If successful, this research will address the biodiesel process to safer and more economically advantageous productions.
Figure 3. Reaction of glycerol to acrolein.
Further analysis are, however, necessary for complex mixtures as the glycerol byproducts, which also contain other FAME residuals, the catalyst, soaps, and others, e.g., when a large excess of sulfuric acid, a competitive reaction of methanol to dimethyl sulfate, however, characterized by a very slow rate, should be considered. Also, methanol may form dimethyl ether, which is highly flammable and may ignite easily, but in the presence of water.39 All of these reactions, however, need a high temperature. Conclusions The paper reports the complete historical analysis of relevant accidents occurring in a biodiesel production plant for 4 years, from 2006 to 2009. The data can be adopted usefully by a health and safety executive (HSE) for the development of appropriate qualitative or quantitative risk assessment. To this regard, it is worth noting that new tools available in the media and on the internet cannot be neglected and that public or commercial databases cannot be the only source of information when lesson-learned and case-history analyses are of concern. As expected, the complete set of accident data shows clearly that the risk of the biodiesel process is mainly related to methanol fires and explosions. However, it should be noted that most of the accidents involving methanol have occurred in backyard utilities, i.e., with total biodiesel capacity lower than about 30 000 tons/year, where discontinuous operations are often adopted, or during maintenance activities.40 On the other hand, the number of plants with a capacity larger than 100 000 tons/year is constantly growing worldwide, for economical reasons. For such large plants, where safety management and workers are more aware of hazards, the number of accidents involving methanol is negligible and
(41) Bournay, L.; Casanave, D.; Delfort, B.; Hillion, G.; Chodorge, J. A. New heterogeneous process for biodiesel production: A way to improve the quality and the value of the crude glycerin produced by biodiesel plants. Cat. Today 2005, 106, 190–192. (42) Di Serio, M.; Tesser, R.; Pengmei, L.; Santacesaria, E. Heterogeneous catalysts for biodiesel production. Energy Fuels 2008, 22, 207–217. (43) Chang, J. I.; Lin, C.-C. A study of storage tank accidents. J. Loss Prev. Process Ind. 2006, 19, 51–59.
(39) Wade, L. G., Jr. Organic Chemistry, 5th ed.; Prentice Hall: New York, 2002. ~ez McLeod, J. E. Human error in biofuel plants (40) Rivera, S. S.; N un accidents. Proceedings of the World Congress on Engineering; London, U.K., July 2-4, 2008; Vol. 2.
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