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48 Safety Aspects of Industrial Dryers
Abstract and Figures
The statistics of industrial accidents show that drying should be regarded as a potentially hazardous operation that has brought a number of reported incidents with serious results for personnel and equipment [1,2]. The data indicate that the accident rate per 105 workers at risk is considerably greater in the food industry than, e.g., in the chemical industry. Approximately 8%-9% of all dust explosions in the food industry is related to the drying operation (Figure 56.1). The other data for the period 1967-1983 in the German sugar industry indicate that drying contributes to 37% of all accidents [3], whereas in the French milk industry an average of four major accidents in spray dryers were reported annually [4]. Based on 89 accidents that happened in 1965-2000, 415 people were injured and 16 fatalities were reported in The Accident Database [5]. It is worth to note that in most cases of spray dryer accident in the food industry re was observed whereas an explosion experienced in <10% [6]. The above reports underline the importance of safety from re and explosion hazards in dryers and in the ancillary equipment.
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... The advantages of superheated steam drying of materials include: an anaerobic atmosphere in the system, obtaining sterile material that ensures a high durability of the fuel obtained and the ease of energy recovery compared to convective air or flue gas drying [3][4][5][6][7]. However, the preparation of material by grinding and storing prior to the drying process, as well as of the dried material, can cause numerous fire and explosion hazards [8]. Inside the drying installation, where the dried material is in contact with steam, the fire/explosion hazard is minimised (this is in accordance with the principles of inherent safety), but outside the internal part of the drying installation, due to the considerable fragmentation of the dried material, it may involve an explosion hazard. ...
... Inside the drying installation, where the dried material is in contact with steam, the fire/explosion hazard is minimised (this is in accordance with the principles of inherent safety), but outside the internal part of the drying installation, due to the considerable fragmentation of the dried material, it may involve an explosion hazard. With the dynamic development of technologies for processing and drying of woody biomass, a significant increase in fires and explosions of dust-air mixtures can be observed, as well as an increase in human, material and environmental losses [8][9][10][11]. In order to prevent and limit the consequences of such incidents, to optimise the superheated steam drying technology and to correctly select innovative explosion protection systems [12][13], knowledge of the fire and explosion parameters of energy willow dust before and after the superheated steam drying process is necessary. ...
In this paper the explosive and fire properties of energy willow dust were experimentally determined before and after drying with superheated steam at temperatures of 120°C, 140°C, 160°C and 180°C. The conducted research has shown that the parameters of the operation of the installation of drying with superheated steam of the energy willow biomass have a decisive impact on the fire-explosive characteristics of the dust produced. The results indicate that the higher the drying temperature, the stronger the probability of ignition of the willow dust cloud, the faster the flame propagation and the higher the explosion intensity. Although the superheated steam drying installation for energy willow biomass is considered to be safe, the probability of occurrence of a fire or explosion events of the biomass dust-air mixture is likely.
... On the basis of reported incidents that involve serious damage to personnel and equipment, drying should be considered as a potentially risky operation, especially in the food industry (8-9% of dust explosions in food processing are directly related to drying operations). Markowski and Mujumdar [1] identified three factors that influence the potential for dust explosion: the process factor, the engineering factor, and the management factor. The first factor is related to the type of material being dried and the physical conditions to which it is subjected; the second factor is related to the plant layout, the equipment employed, and its standards; and the third factor comprises risk assessment and examination of hazards associated with the drying operation. ...
... In most installations, vents are situated on the wall of the cylindrical portion of the dryer, before the beginning of the cone, and in certain cases they still exist in dryers with pressure reliefs at the top of the chamber cylinder. The location of the vents in the top of the drying chamber presents two drawbacks: (1) it is a risk for workers who carry out certain routine maintenance activities when equipment is in operation; and (2) it is costly, since this part is where the atomizer is located. In addition, experiments conducted in a Norwegian 236 m 3 silo demonstrated that when the vent is located in the wall of the silo, it helps to lower pressures in comparison to when it is located on the roof (under the same experimental conditions). ...
Venting systems are commonly employed to minimize damage and/or losses caused by dust explosions in spray dryers. At present, the two most commonly used vent-sizing standards are the American standard NFPA-68 (2007) and the European standard EN-14491 (2006). The work described in this paper has two main aims. The first aim is to compare the results of the venting area as required by the two aforementioned standards for conventional milk drying when dealing with three products commonly treated in the dairy industry: buttermilk, powdered milk, and full-fat milk. The vent area was calculated by taking into account the reduced pressure (Pred) and the geometry of the spray dryer (aspect ratio of the vessel length/diameter). In addition, the results obtained as per NFPA-68 were also corrected on the basis of turbulence intensity and partial volume. The second aim is to apply the two standards to the real case of a spray dryer in a dairy processing plant located in the south of the province of Lugo (NW Spain). The results showed significant differences between the two standards in that the vent areas obtained following the criteria of EN-14491 were always higher than those obtained with the American standard (except for the cases in which L/D = 1).
... Selection of the dryer and optimisation of drying conditions requires in-depth understanding and quantification of the biomass material, requirements of energy conversion, and the drying process [2,3]. These aspects can affect the level of fire and/or explosion risk and selection of fire-extinguishing method [4]. If substances that do not pose fire and/or explosion risk are dried, the criterion for using the particular drying method depends on the expected drying efficiency, area available for building-up the drying system, availability of the preferred drying medium, unit drying costs etc. ...
Studying of the drying kinetics and fire and explosion characteristics of biomass is a fundamental aspect in designing material drying systems based on superheated steam, and proper selection of explosion and fire protection systems. The paper presents studies on osier willow (Salix viminalis) drying kinetics under isothermal conditions (120 °C, 140 °C, 160 °C, 180 °C) and their use for process calculations of the proposed one-dimensional model of superheated steam pneumatic dryer. The model included empirical parameters in order to describe the volumetric heat transfer and specific surface of the wood particles of irregular shapes.
In addition, a 20-L spherical explosion chamber, Hartmann tube apparatus, hot plate apparatus and Godbert-Greenwald furnace apparatus were used to investigate the explosion and fire characteristics of willow dusts. Based on the studies it was confirmed that the operating parameters of superheated steam drying system for woody biomass have a major impact on the explosion and fire characteristics of the formed dust. The results indicate that the lower the moisture content in the willow, the more likely the dust cloud to be ignited, the faster the explosion flame propagation, and the greater the explosion severity. Despite the fact that the superheated steam drying system for woody biomass is considered safe, the probability of occurrence of fire or explosion incidents related to the dust and air biomass mixture is high. The maximum temperature for superheated steam drying equipment covered with up to 5 mm thick layer of osier willow dust should not exceed 215 °C.
... toxic organic solvents such as methanol or chloroform which as frequently used in nonfood extraction processes) (Giusti and Jing 2008). Further, the aqueous method may present cost and operational advantages in an industrial setting (e.g., elimination of flame proof facilities and any fire hazards potentially caused by alcohols during spray drying) (Markowski and Mujumdar 1995;Sun-Waterhouse et al. 2008). The legal definitions of fruit juices or fruit juice concentrates require the use of physical methods and/or aqueous processes for extracting juice (Title 21, US Code of Federal Regulations 73). ...
This study aimed to establish an efficient route for converting blueberry waste material (BWM) into antioxidant rich powders. Extracts were produced from BWM by an aqueous method using water acidified with citric acid, in the absence or presence of Pectinex Ultra SP-L and Cellubrix. All BWM extracts contained antioxidants including phenolic acids, anthocyanins, and flavonoids (total phenolic content (TPC) 3655–4369 mg gallic acid equivalent (GAE) and total anthocyanin content (TAC) 219–296 mg cyanidin-3-glucoside equivalents (CyGE) per 100 g dry extract). Extractions at 50 °C yielded higher TPC and TAC but lower vitamin C and pectin contents than extractions at 20 °C. Spray-drying BWM extracts produced at 50 °C (no enzymatic treatments) and an encapsulant (alginate or inulin) at an inlet temperature 150 °C and feed temperature 50 °C yielded powders with desired dark purplish blue color, water activity (0.25–0.33), flowability, reconstitution time (23–46 s in water or milk), TPC (25–30 mg GAE/g), TAC (17–20 mg CyGE/g), storage stability, and Bifidobacterium-boosting properties. Enzymatic pretreatments of BWM did not confer any advantages in preserving antioxidants in powder products, suggesting that some intrinsic BWM components (e.g., pectins) may play an important role in the encapsulating process. The use of alginate as the encapsulant/drying aid afforded higher powder yields, superior protection of antioxidants, better stability over a prolonged storage or elevated temperature storage, greater retention of TPC/TAC under simulated gastrointestinal conditions, and greater Bifidobacterium-boosting effects, compared to powders prepared using inulin. Thus, simple aqueous extraction methods and spray-drying technology hold enormous promise for producing antioxidant-rich powders from blueberry processing by-products or waste.
... Such losses can be reduced by recirculating a proportion of the drying air. However, this requires extensive modifications to be made to the dryer (Gong et al. 2011), and for a pneumatic dryer can increase the risk of dust explosion (Rotstein and Crapiste 1997;Markowski and Mujumdar 2014). Another way to reduce exhaust heat losses is by reducing the air mass flow rate (Kudra 2009). ...
In sub-Saharan Africa, cassava is grown by smallholder farmers and is the principal source of calories for the local population. However, the short shelf life of cassava associated with poor infrastructure in the region results in significant postharvest losses. The expansion of small-scale cassava processing could reduce these losses, but the availability of drying equipment suitable for use in such operations is limited. The objective of this research was to contribute to the development of cassava dryers suitable for use by smallholder farmers. A tunnel dryer and a pneumatic dryer being operated in Tanzania were evaluated using mass and energy balance analysis. It was found that the energy efficiency of the tunnel dryer was 29% and of the pneumatic dryer 46%. For the tunnel dryer, most of the heat losses were through unsaturated exhaust air, while for the pneumatic dryer, most losses were through radiation and convection.Practical ApplicationsIn this study, a tunnel dryer and a pneumatic dryer suitable for use by smallholder farmers were evaluated during processing centers' usual cassava drying operations. The sources and extent of heat losses were identified, and then guidelines developed on how to reduce such losses. For both dryer types, improvements to the thermal insulation used could reduce heat losses to the ambient. For the tunnel dryer, decreasing the air mass flow rate by 57% would help to minimize exhaust heat losses without producing condensation inside the unit. For the pneumatic dryer, air mass flow rate could be reduced by 9%, improving energy performance without having a negative impact on the pneumatic conveying of the product. Those two modifications would be easy to implement and represent a significant contribution to the development of small-scale cassava drying technology.
Hazardous explosive atmospheres caused by dust may occur when powdered combustible material is present in an enclosed area. In general, dust explosions take place when high enough concentrations of combustible powdered dust particles are present in the atmosphere with another suitable gaseous medium such as oxygen. Because the manipulation of these substances in industrial activities entails a risk that could have devastating consequences (including serious injuries or loss of life, as well as significant damage in installations), the risk assessment to eliminate or control the risk of explosive atmospheres in the workplace is a challenging task. In European countries the assessment of these risks is evaluated using the ATEX Directives as per European legislation. However, using this legislation to assess the risk of explosion in powder atmospheres presents difficulties due to the variety and complexity of possible explosive atmospheres and the extremely high variety of ignition sources involved. Therefore, risk evaluation becomes a difficult and time-consuming task.
Drying is an especially hazardous operation. To make this operation safer, it is critical to understand the hazards associated with decomposition, fire, and explosion including worst-case scenarios resulting from operational and control failures.This article describes the tests and standards required to prevent accidents when handling bulk powders and dusts. The outcome of these tests and standards is to quantify the risks for normal and upset conditions and to implement adequate safeguards to prevent accidents. © 2012 American Institute of Chemical Engineers Process Saf Prog, 2012
Food dehydration, a traditional method of food preservation, is also used for the production of special foods and food ingredients,
and for the utilization of food plant wastes. A wide variety of industrial food drying equipment is used, developed mostly
empirically, but continuously improved by recent advances in drying technology and food engineering. In addition to the basic
process engineering requirements, food dryers must meet the strict standards for food quality and food hygiene and safety.
Fire and explosion hazards as well as safety measures encountered in drying processes are presented. Those data were used to work out a simplified layer of protection analysis (called dLOPA). In the developed method, necessary tools based on the matrix concept were presented. It concerns the assessment of an effective ignition source probability and severity of a possible consequence of dust explosion. A step-by-step procedure for dLOPA analysis is presented. This method may find application in risk assessment for workers exposed to explosive atmosphere (ATEX Directives).
Following is the continuation of the list of titles and authors: Reliability Engineering - A Rational Technique for Minimizing Loss. By B. A. Buffham, D. C. Freshwater, And F. P. Lees. High Integrity Protective Systems. By R. M. Stewart. Design for Loss Prevention - Plant Layout. By H. G. Simpson. Loss Prevention Aspects in Process Plant Design. By P. L. Klaasen. Fire Detectors for Use on Chemical Plants. By B. G. Steel. Review of Recent Advances in Fire Extinguishing Chemicals. By W. W. Harpur. Dry Chemical Fire Extinguishing Systems and Installations in Chemical Industries. By F. Emmrich. Relief Venting of Dust Explosions in Process Plant. By K. N. Palmer. Ignition and Burning of Dispersions of Flammable Oil in the Form of Pools. By D. G. Wilde and G. E. Curzon. Safe Dispersal of Large Clouds of Flammable Heavy Vapours. By E. M. Cairney and A. L. Cude. Protective Measures and Experience in Acetylene Decomposition in Piping and Equipment. By Herbert Schmidt.
The author examines the factors involved in dust explosions, the minimum conditions under which an explosion can occur and the measures to be taken to prevent it, or at least limit the damage caused, by using various apparatus and devices that minimise the consequences of an explosion. Tests carried out on small-scale models in the laboratory, together with recent tests on medium-sized real silos equipped with anti-explosion devices, and examination of the devastating consequences of explosions in large silos and plants for storing, handling and processing combustible products, have enabled certain conclusions to be drawn that should be taken into account when designing an installation of this type. Finally, the author reviews the range of apparatus and devices available on the market for eliminating or at least minimising the risk of explosion.
A description is given of the essential features of dryer-blenders. 5 refs.
A fire, explosion, or other catastrophy can be a devastating experience in any industrial environment. In the chemical industry, this is a special concern given the history of the industry. The nature of the risks involved in the chemical and allied industries is characterized by: fast-changing technologies; the development of large, so-called world-scale plants; a huge capital investment base; and the large earnings potentials associated with single train or very compact facilities. The following aspects are analyzed - loss experience; frequency; major explosions; major fires; risk benefit.
The factors that affect explosions - ignition sensitivity and explosion severity - are discussed. Methods for detecting and preventing dust explosions are also outlined.
The intension of this research is to evaluate explosion suppression system performance against violent gas and dust explosions in a confined volume of 250 m**3. It is found that a system using a large number of small explosion suppressors is more effective than one using fewer larger suppressors, though the latter is cheaper.
An analysis of the fire and explosion risk in spray dryers is presented by consideration of the potential ignition sources and the flammability of materials (powders, dust clouds, vapours). The means of achieving safe operation (e.g. avoidance of ignition sources, inert atmospheres, explosion venting, suppression and containment) are discussed, compared and applied to spray dryer systems using flammable and non-flammable liquors.
The drying of powders can present a decomposition, fire, or dust explosion hazard and the manufacturers and users of dryers must consider the nature of the potential risk and the most appropriate basis for safe operation. The existing methods for dust explosion risk evaluation and this procedure for the characterization of exothermic decomposition provide a means whereby safe drying conditions can be specified for multi-product industrial dryers. In certain cases, safety can be based on the avoidance of decomposition and ignition, but in the majority of situations the control of the atmosphere to make it non-flammable or the use of explosion protection (venting, containment, suppression) will be required.