The fermentation industry includes the production of malt beverages (beer); wines; brandy and brandy spirits; distilled spirits; and the secondary products of all of these industries. The most commonly produced distilled spirits for beverage purposes include whiskies, gins, vodkas, rums and brandies.
Whiskies are produced from fermented grain mashes and aged. Vodkas are produced from fermented grain mashes, but are not aged. Gins generally are produced from the fermented product, grain neutral spirits
(GNS), to which either botanical extracts and/or flavors are added to the GNS and bottled, or dried botanicals (e.g., juniper berries) are added to the GNS to extract their oils and then distilled. Rums are made from fermented sugar cane products, such as molasses. Gins and rums may be aged in barrels. Brandies are distilled from wine or other fermented fruit juices, and are generally aged in barrels.
Distilled spirits production (e.g., whisky, vodka, or gin) may produce secondary products, such as distillers dried grains used as livestock feed.
Distilled spirits can be produced by a variety of processes. Typically, whisky production utilizes malted grains which are mashed and fermented to produce an alcohol/water solution that is distilled to concentrate the alcohol. This is not necessarily true for production of other distilled spirits, such as vodka, rum and brandy. The concentrated alcohol is usually aged in wooden barrels to provide natural color and impart flavor and aroma. Recognizing that not all distillers employ identical techniques and materials, this section attempts to provide a generic description of distilled spirits (distillery) operations.
Grain Handling & Preparation
Distilleries utilize premium cereal grains, such as hybrid corn, rye, malted barley, and wheat, to produce the various types of whisky and other distilled spirits. The grains have particular specifications, especially with regard to the elimination of grain with objectionable odors which may have developed in the field or during storage, handling, or drying at the elevators.
Grain receiving, handling, and cleaning are potential sources of particulate matter (PM) emissions. Grain is generally received in either hopper railcars or trucks. Grain handling is the transfer from the unloading pit by pneumatic conveyor system, auger system, and bucket elevators to and from the grain storage silos. Although it usually has been subjected to a cleaning process at the elevator, the grain may be subjected to additional cleaning, which may include a series of vibrating screens that sift out foreign materials and magnetic separators used to remove any ferromagnetic items. Dust collectors and air jets may be used to remove light materials and aid in the control of PM emissions.
Milling, which breaks the outer cellulose protective wall around the kernel and exposes the starch to the cooking and conversion process, can be accomplished by several milling methods. For example, hammer mills use a series of hammers rotating at 1,800 to 3,600 rpm within a close-fitting casing. These hammers shear the grain to a meal that is removed through a screen with different mesh sizes for various types of grain. Cage mills use a series of counter rotating bars at high speed to grind the grain by impact.
Roller mills use a series of close tolerance serrated rollers to crush the grain. Distillers require an even grind, generally with a particle size as small as can be physically handled by the facility.
Processes require heat. Other compounds can be generated in trace quantities during fermentation including ethyl acetate, fusel oil, furfural, acetaldehyde, sulfur dioxide, and hydrogen sulfide.
The mashing process consists of cooking (gelatinization) of the grain in water to solubilize the starches from the kernels and converting (saccharification) of the starch to "grain sugar" (primarily glucose and maltose). In general, cooking can be carried out at or above atmospheric pressure in either a batch or continuous process. During mashing, trace VOC emissions may result from constituents in the grain.
Small quantities of malted barley are sometimes added prior to grain cooking. After partial cooling, conversion of the starch to sugar is accomplished by adding barley malt and/or enzymes (from other sources) to the cooked grain. The mash then passes through a noncontact cooler to a fermenter. Between the mashing and fermentation, the process generally is closed during cooling, with no emissions. Distillers may vary mashing procedures, but generally conform to basic principles, especially in the maintenance of sanitary conditions.
Fermentation, which usually lasts 3 to 5 days for whisky, involves the use of a yeast to convert the grain sugars into ethanol and carbon dioxide (CO2). The converted grain mash is cooled prior to entering the fermenter or tank and inoculated with yeast. It is common practice to dilute the hot grain mash to its final solids concentration by adding backset stillage and/or water. Backset is liquid stillage which is screened or centrifuged from the distillation "beer still bottoms." The use of backset provides water conservation, nutrient supplements, pH adjustment of the fermentation, and some flavor components (e.g., sour mash).
The fermentation process varies slightly for the production of other distilled spirits. For instance, rum fermentations takes 1 to 2 days. In rum production, black strap molasses is the source of fermentable sugars and is stored in tanks prior to fermentation. The black strap molasses also is not "mashed" (i.e., cooked) prior to being diluted with water to obtain the proper concentration of fermentable sugars.
Congeners are flavor compounds which are produced during fermentation, as well as during the aging process. These congeners include trace aldehydes, esters, and higher alcohols (i.e., fusel oils). Lactic acid bacteria (lactobacillus) may simultaneously ferment within the mash and contribute to the overall whisky flavor profile. On rare occasions lactobacillus may provide some pH control. On other occasions, the addition of sulfuric acid, though rarely used, may result in trace hydrogen sulfide emissions from the fermentation tank.
In whisky production, significant increases in the amount of yeast consumed occur during the first 30 hours of fermentation, when over 75 percent of the carbohydrate (sugar) is converted to ethanol and carbon dioxide. Many fermentation vessels are equipped with agitation and/or cooling means that facilitate temperature control. Fermentation vessels may be constructed of wood or metal and may be open or closed top.
The final fermented grain alcohol mixture, called "beer," is agitated to resuspend its solids and may be transferred to the "beer well" storage vessel for holding until it is pumped to the "beer still." Distillers use mechanical or air agitation during transfer and storage to prevent settling of solids. In the instance of air agitation, trace amounts of aldehydes may be produced. The beer passes from the beer well through a preheater where it is warmed by the alcohol vapors leaving the still and then enters the still for distillation.
The beer still vapors condensed in the preheater generally are returned to the beer still as reflux.
The distillation process separates and concentrates the alcohol products from the fermented grain mash. In addition to the alcohol and congeners, the fermented mash contains solid grain particles, yeast cells, water-soluble proteins, mineral salts, lactic acid, fatty acids, and traces of glycerol and other trace congeners. Whisky stills are usually made of copper, especially in the rectifying section, although stainless steel may be used in some stills. In a general whisky distillation process using a beer still, the whisky separating column consists of a cylindrical shell having three sections: stripping, entrainment removal, and rectifying. The stripping section contains approximately 14 to 21 perforated plates, spaced 56 to 61 cm (22 to 24 inches) apart. The fermented mash is introduced at the top of the stripping section and descends from plate to plate until it reaches the base where the stillage is discharged. Steam is introduced at the base of the column, and the vapors from the bottom of the still pass up through the perforations in the plates. Whisky stills are usually fitted with entrainment removal sections that consist of a plate above the stripping plate to remove fermented grain particles entrained in the vapor. Distillation columns operate under reflux (sealed) conditions and most vapors are condensed and collected, although small amounts of noncondensable gases will be emitted to the atmosphere. The rectifying section contains several bubble cap or valve rectifying plates in the top section of the still that produce distillates (ethanol) up to 190E proof.
The diameter of the still, the number of stripping and rectifying plates, capacity of any doubler, and proof of distillation are factors that can contribute characteristics to a particular whisky. The doubler is a type of pot still that is used to redistill the distillate from the beer still to enhance and refine the flavors desired in a specific whisky. Following distillation, the whisky, at high proof, is pumped to stainless steel tanks and diluted with demineralized water to the desired alcohol concentration prior to filling into oak barrels.
Grain & Liquid Stillage
At most distilleries, after the removal of alcohol, still bottoms (known as whole stillage) are pumped from the distillation column to a dryer house. Whole stillage may be sold, land applied (with appropriate permitting), sold as liquid feed, or processed and dried to produce distillers dried grains (DDG). The DDG consists of proteins, fats, minerals, vitamins, and fibers which are concentrated threefold by the removal of the grain starch in the mashing and fermentation process. Distillers' secondary products are divided into four groups: DDG, distillers dried solubles (DDS), DDG with solubles (DDG/S), and condensed distillers solubles (CDS).
Solids in the whole stillage are separated using centrifuges or screens. The liquid portion "thin stillage" may be used as a backset or may be concentrated by vacuum evaporation. The resultant syrup may be recombined with the solid portion or dried separately. This remaining mixture is then dried using one of a variety of types of dryers (usually steam-heated or flash dryers). The majority of DDG are used in animal feed, although increasing quantities are being sold as food ingredients for human consumption due to its nutrient and fiber content.
In the aging process, both the charred oak barrel in which beverage alcohol is stored and the barrel environment are key to producing distilled spirits of desired quality and uniqueness. The aging process gives whisky its characteristic color and distinctive flavor and aroma. Variations in the aging process are integral to producing the characteristic taste of a particular brand of distilled spirits. Aging practices may differ from distillate to distiller, and even for different products of the same distiller.
After the whisky has completed its desired aging period, it is dumped or pumped from barrels into stainless steel tanks and reduced in proof to the desired alcohol concentration by adding demineralized water. The diluted whisky is processed and filtered. Following a filtration process the whisky is pumped to a tank, proof adjusted, and bottled.
The major hazard in respect of the distillery stems from the production of flammable organic compounds like ethanol. Trace elements of ethyl acetate, isobutyl alcohol etc are also present. The hazards arise from leaks in the tanks themselves, casks and ancillary equipment such as transfer pumps, pipe work and flexible hoses, all of which can release significant quantities of liquid on failure. A vapour cloud explosion may be possible depending on the size of the release, the spillage surface, and the presence of confined volumes or adjacent structures that produce flame acceleration.
The grain processing section also has a considerable fire hazard due to the production of flammable grain dust other particles generated in the process.
Old distilleries have wooden constructions in respect of intermediate floors and racks which can cause combustion. Also wooden cask stores have considerable fire load and can supply enough fuel to sustain a fire.
Whisky maturation warehouse sites sometimes hold a variety of other hazardous materials, which may need to be considered in the report. These include a natural gas supply to a boiler, a LPG cylinder, dangerous powders and/or toxic substances in tanks or drums.
Ethanol is a highly flammable liquid with a relatively low flash point, -21 ºC – the flash point is dependent on the alcohol concentration. There is a concern that evaporating ethanol could pose an explosion hazard in bonded warehouses and in stills rooms. The vapour-air density of ethyl alcohol is 1.6 times that of air. Ethyl alcohol vapours are invisible, and the distance they will travel is not always predictable. Testing carried out by the Distilled Spirits Council of the United States indicates that beyond 0.5 metres from the source, vapours are generally less than 25% of the LEL and beyond 1.65 metres they are usually negligible.
However, from a study of the major losses involving distilleries, it has been assumed that the chances of fire and subsequent explosion originating in the storage areas are far too higher than the process areas. The natural ventilation is an integral part in the maturing process of the whisky, lending it a particular quality, which will be different from region to region. It has been assumed that a typical air change rate is of the order of ten air changes per hour. It would appear that there is a low probability of an explosion due to the ignition of an ethanol/air mixture. The evaporation rate of ethanol at ambient conditions of about 25 ºC is too low; the natural ventilation would almost certainly be able to dilute the gas cloud ethanol concentration down to well below its lower flammability limit. However if there is a possibility of recirculation zones or stagnant regions, where the gas cloud could, potentially, become enriched so as to fall between the lower and upper flammability limit. Explosions could be triggered in the event of an ignition due to any source of electricity like sparks, static electricity discharge etc.
Studies conducted in respect of distillery accidents have indicated that in most of the cases a fire would precede an explosion - radiation and convection enhancing the vaporisation of ethanol, destroying casks and/or structures leading to further spillage and subsequent ignition of the explosive mixture.
The following are five mechanisms for the production of a flammable mixture
A fire in an adjacent building, if unsuccessfully checked, could lead to a temperature rise in the warehouse. The higher temperature could then result in an increased ethanol evaporation rate and, thus, to the formation of a gas cloud.
The scenario of spillage or leakage in the stills room is more complex. The temperature in the copper distillers is well above 25 ºC, so that the surface temperature will also be relatively high, but well below the auto-ignition temperature of ethanol, which is around 430 ºC. It is therefore not possible for the hot surface to auto-ignite the ethanol/air mixture. A hot surface would lead to an increased evaporation rate of ethanol but would not necessarily lead to an explosion, if the fire were in the vicinity of the hot surface, as a relatively small fire would ensue. However, a flammable gas cloud could form if the fire was located a substantial distance away from the hot surface.
The production of an explosive mixture due to the heating up of a storage tank or cask has not been studied in detail. It is noted, however, that this scenario is a likely one. The oak used to manufacture casks is a good insulator, so the temperature rise in the whisky would be relatively slow. A possible outcome of the flame engulfment of a cask is that the metal hoops, with high thermal conductivity, which hold the oak staves together, would heat up first. It is interesting to note that the oak has nearly four times higher thermal expansion coefficient than the carbon steel used for the hoops, which means that the staves and the cask ends would expand more than the metal hoops for a given temperature difference. The temperature in a flame could be of the order of 500 °C to 1000 °C, at which steel would begin to soften. One possible failure mechanism would be that the steel hoops snap or get dislodged due to the large difference in thermal expansion between the wood and the steel, thus relieving the pressure in the cask. The ethanol vapour would then ignite.
As discussed, the generation of flammable vapours and their subsequent fire and explosion is one of the likely scenarios in distilleries. The air change rate and the airflow pattern in the room would be two important factors in deciding whether an explosive mixture would form and where it would be located.
Explosion venting as per is required in the area or room where the distillation of ethyl alcohol takes place. The concentration of alcohol vapour in the air within a distillation still usually exceeds the UEL. Any vapours escaping from a still may become explosive when mixed with air, however, extensive testing has shown that the vapour dissipates to safe concentrations within 1 metre of the point of release.
Storage tanks, wooden vats, aging barrels, drums or containers used to store or process alcohol must be designed, fabricated and tested in accordance with good engineering practices to withstand the anticipated maximum operating pressure or temperature. Storage tanks used for ethyl alcohol may be steel or stainless steel (for purity). Good engineering practices are provided in a guide recommended by The Distilled Spirits Council of the United States Inc., entitled "Recommended Fire Protection Practices for Distilled Spirits Beverage Facilities."
Since exposed steel supports do not have a 2 hour fire-resistance rating they require protection as do the timber supports for tanks. Automatic sprinklers have proven to be an effective means of achieving the required protection provided there is enough space under the tank to install them.
The design of the normal and emergency venting should be such that accumulation of flammable vapours inside the building is prevented. New tank installations can achieve this by directing breather vents and emergency vents, equipped with flame arresters or pressure/vacuum valves, to the outside of the building.
If ventilation design principles are applied to the building ventilation, venting into the building space may be acceptable for existing installations. Such measures include, but are not limited to: installation of automatic sprinklers throughout the tank room and under any raised tanks greater than 1.2 metres in diameter; classification of electrical equipment and wiring according to the zone classifications; provision of adequate natural or mechanical ventilation meeting the objectives.; and training of personnel in safe operating procedures.
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