Safety issues related to the maintenance operations in a vegetable oil refinery: A case study
Introduction
An increase of the edible oil industrial production by seed extraction is expected in the next years due to its growing consumption and request (FAO, 2013). Therefore, a wider diffusion of industrial facilities for vegetable oil processing and refining will consequently occur, also close to residential areas.
A critical phase of the edible oil production chain is the final refining aimed at removing free fatty acids, which, in too high concentrations, may cause the rancidity of the oil (Akterian, 2011, Bhosle and Subramanian, 2005, Calliauw et al., 2008, Carmona et al., 2010, Cavanagh, 1976, Cuevas et al., 2009, Keurentjes et al., 1991, Martinello et al., 2007, Sullivan, 1976), and other minor components such as phospholipids, pigments, proteins, oxidation products and the possible residual content of the solvent used for the extraction process. This stage of the production chain is crucial for the quality enhancement of the final product. Despite the extremely limited amount of hazardous substances in this type of facilities, safety and environmental criticalities associated to the refining process were discussed in previous works (Landucci et al., 2011, Landucci et al., 2013). In particular, the possibility of accidental combustion associated to the accumulation of the extraction solvent (typically hexane) was discussed, also considering the recent accidents occurred in Italy (La Repubblica, 2006) and Spain (El Economista, 2007), that caused several fatalities among maintenance operators.
Another potential safety issue associated to maintenance operations in vegetable oil refining plants is related to the accumulation of unwanted solid residuals downstream the chemical treatment of the crude vegetable oil, which consists of degumming, neutralization, washing, drying and bleaching (Gunstone et al., 1994, Loft, 1990, Mag, 1990, Santori et al., 2012, Shahidi, 2005). In particular, in this latter treatment, bleaching earths and activated carbon are mixed with the oil in order to remove pigments; next, the slurry is filtered obtaining a liquid phase which is further processed in the deodorization. This operation consists of high temperature steam stripping in a packed column. This stage is aimed at removing the residual acid compounds at low pressure (2 mbar) and high temperature (200–250 °C). For prolonged periods of operations or due to poor maintenance, residuals of filtration may accumulate in the column packing causing loss of efficiencies (pressure drops and non-uniform liquid distribution) and generating potentially hazardous situations (Kister, 2003). In particular, it is well known that in case combustible material residuals are distributed on column packing or, more in general, in equipment internals, one of the hazards is related to the possible auto-ignition (Davie et al., 1993, Plellis-Tsaltakis, 2012); in some cases, parts of metal structured packing may ignite, heating the equipment wall to temperatures able to compromise the structural integrity of the process equipment (Ender and Laird, 2003, Mannan, 2003, Roberts et al., 2003).
In the present work, an incident occurred in an industrial vegetable oil refinery at the deodorization column is presented to evidence this safety criticality in a process in which no combustible materials were expected to accumulate and cause hazardous situations. The incident took place in the night between September 26th and 27th 2012, during a maintenance shut down. A fire was generated by the auto-ignition of the solid residuals in the column packing. No fatalities or injuries were reported, but the column was strongly damaged and removed from the plant.
A specific experimental characterization of the solid residual, sampled both from the damaged and undamaged parts of the column, was carried out in order to reproduce the column internal conditions at the moment of the incident and to verify the possibility of auto-ignition of the residual. On the same time, steel samples taken from the column wall, both from the damaged and undamaged parts, were subjected to metallographic and hardness analysis. Moreover, samples collected from the undamaged parts of the column were heat treated at different times and temperatures and their microstructure and hardness were compared with that of the damaged parts in order to obtain information about the incident duration and temperature reached during the combustion phenomenon.
Section snippets
Description of the incident
The incident occurred in the deodorization column of SALOV S.p.A. refinery during the night between September 26th and 27th 2012. The refinery is located in Massarosa (Italy), between vulnerable residential areas of Viareggio and Massarosa as shown in Fig. 1a.
The facility, which covers a total surface of about 100,000 m2 and features an overall storage capacity of 8500 tons, produces 1.3 × 105 m3 per year of different oil types.
A schematic representation of the process is shown in Fig. 1b, in
Investigation methodology
Fig. 3 reports the flowchart that summarizes the methodology followed for the incident investigation. The analysis was aimed at evaluating the entity of the mechanical damages on the column and at characterizing the solid residual accumulated in the packing.
As shown in Fig. 3, the structural damage was evaluated by a specific metallurgical analysis, carried out both on the damaged and undamaged parts of the column (Section 3.2.1). The analysis allowed to obtain an indication of the maximum
Evaluation of the thermo-mechanical damage
Fig. 4, Fig. 5 respectively show micrographs of B and D samples collected from the column wall. For both samples, it is apparent an inhomogeneous microstructure induced by the rolling processes with smaller grain size in the central segregated region and larger ones in areas close to the surface. However, D samples show larger grain size than B samples in all the wall section regions. Since the inner surface is the most damaged area, Vickers hardness and grain size measurements were performed
Conclusions and recommendations
The present work focused on hazards which may affect edible oil processing plants during maintenance operations. In particular, an incident occurred in an industrial vegetable oil refinery was presented in order to highlight this safety criticality. The incident took place in the night between September 26th and 27th 2012, during a maintenance shut down, and was associated to the ignition of the solid residual in a packed stripping column. No fatalities or injuries were reported, but the column
Acknowledgments
The authors gratefully acknowledge financial support received from Regione Toscana (Bando Unico R&S n.2009DUA/526090469/1).
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