Technology description
Alcohol fermentation is a process in which sugar containing biomass is converted to alcohol, e.g. ethanol by the metabolism of microorganisms. The fermentation is usually anaerobic, but also aerobic conditions can be feasible. Fermentation processes can be batch, fed-batch or continuous processes. (Nag 2007)
Raw materials with high sugar content, such as corn and sugar beet, are the most suitable for the fermentation process. In addition, lignocellulosic raw materials, like wood and straw can be also utilized in order to produce ethanol. However, lignocellulosic raw materials require acid or enzyme pre-treatment, because cellulose and hemicelluloses need to be converted into sugars that the microbes can use. Notwithstanding, lignocellulosic raw materials are considered to be more sustainable since they do not use food resources. (Nag 2007)
The conventional fermentation process consists of hydrolysis, fermentation, separation and purification steps. If the process uses lignocellulosic raw materials, milling and acid or enzymatic hydrolysis is required. After pre-treatment, sugars go through the fermentation process. The produced alcohol is removed from the process at concentrations around 6%, because ethanol concentration at 15% and above start to be toxic for the microbes. In the end, ethanol is enriched to 99% bioethanol. (Nag 2007, MicrE 2011)
The most conventional process to separation of water and ethanol is distillation. Distillation is an expensive and energy intensive solution. Current research efforts concentrate at low energy separation processes, such as membrane processes, in particular pervaporation. (Nag 2007)
Available commercial size for a bioethanol plant varies very much, but annual ethanol yield up to 100 m3 can be reached (Vogel 1983). The alcohol produced can be used either in CHP units in order to produce electricity and heat or as a fuel for vehicles. Solid residues from the fermentation process can be used as fertilizer or as animal feedstock. (Scragg 2006)
Installation a fermentation plant starts with defining the raw materials. It is necessary to know what goes into the fermentation process, since it defines the needed alcohol producing microorganisms. The properties of raw materials determine also the need and type of a pre-treatment process, for instance milling and hydrolysis. (Nag 2007)
Once the data for the input is defined, the supplier for the ethanol plant is considered. Agreements with other raw material suppliers can also be done. (Tavitsevainen 2006)
An essential part of the installation of a fermentation plant is having a construction permit for it. Land use planning regulation and city plans will also need to considered. Landscape permission may also be necessary in some countries. (Ministry of Environment 2011b)
Environmental permits, including Environmental Impact Assessment is essential to complete before further planning of a plant. Legislation related to water and water supply may also be considered, if the process water is taken from a lake or river. In addition, regulations related to wastes and emissions to the atmosphere and possible noise emissions need to be taken into account. (PÖYRY 2006)
Permits to execute both the process and the production of high concentrate ethanol are required. Ethanol is a dangerous chemical with flammable properties, therefore, requirements, standards and permissions related to processing, transportation and storing of a hazardous chemical and waste need to be followed. (PÖYRY 2006)
A rescue plan, hazard identification and risk evaluation are also preferable to compose. Regulations related to fire safety and stability of the building is necessary to follow, in order to ensure a safe working environment. (PÖYRY 2006)
The end product, bioethanol, can be sold to fuel companies after completing agreement with them. In addition, the possible utilization of solid and liquid by-products for animal feedstock will be required. Installation of a bioethanol plant requires also legislative systems and regulations, which are linked with installation of pipes, storage and transportation. (PÖYRY 2006) (EPA 2007)
Ethanol is a harmful and highly flammable compound, which is in liquid phase in normal temperature and pressure conditions. This organic compound is also toxic for humans and animals, especially in high concentrations. Because of these properties, ethanol must be handled, stored and transported properly. Following safety requirements, considering ventilation and keeping sources from ignition away prevents accidents. Bringing a safety professional to check and evaluate the safety of the process is necessary. (Safety data 2011)
If there is grinding or milling process as a pre-treatment process of raw material, respiratory protection is essential due to high dust concentrations. The possibility of high CO2 concentrations near the fermentation tanks requires also the use of respiratory protection. In addition, coolant compounds such as ammonia, glycol, propane to cool down the fermentation process are considered to be hazardous. (Liao & Saffron 2008)
Malfunction causing excess pressure and temperature in the distillation column can be safety hazard. In case of uncontrollable process circumstances, the column can broke and release highly flammable ethanol into air. The cooling system must work properly, otherwise the temperature of the process will rise rapidly. As well, leaks from distillation column can be hazardous. (Tham 2011)
Acidic or chemical hydrolysis as a pre-treatment process for starchy and lingocellulosic material can cause safety hazard depending on the type and concentration of the compounds. Sulfuric acid is a conventional chemical compound to hydrolyze starchy feedstock, and it poses a safety hazard being highly toxic and corrosive. Furthermore, some enzymes can also be hazardous. (Nag 2007)
Adequate pressure gauges and valves are necessary in the pressurized CO2 and fermentation vessel to avoid accidents. Also leaks from pipes and tanks can cause safety problems. (Liao & Saffron 2008)
We can say that fermentation is quite a safe process, although the safety issues depend highly on the process type, which is used. However, if the process uses conventional yeast, saccharomyces cerevisiae, as an ethanol producer, toxic and hazardous affects related to that certain microbe are rather minimal. There are still cases when fermentation process can include genetically modified microorganisms. This kind of organisms can be considered a hazard to humans and the environment. (Nag 2007)
It is necessary to know what micro-organisms are working in the process, because they also define the material input for the process. Some micro-organisms cannot use some specific sugar in their metabolism, which can restrict the fermentation process. The length of retention time also affects greatly the ethanol yield. (Nag 2007)
Possible inhibitors for the process are usually ash, furfur, levulinic acids and both aromatic and inorganic compounds. Antibiotics-containing input can restrict or even kill the micro-organisms. (MicrE 2011)
In a fermentation process, the process conditions have to be optimal for microbial growth and action. At first, the temperature should be appropriate for the used microbe. Lack of possible coolant compounds can raise the temperature of the process significantly. In addition, the water content of a growth medium has to be right. (Scragg 2005, 52)
Lack of nutrients can cause inefficient ethanol yield. Micro-organisms need several nutrients and trace elements, such as carbon, hydrogen, phosphorus, sulphur, vitamins, kalium and calcium. Adequate pH-level is also vital for fermentative micro-organisms. Accurate pH-level can be controlled by adding ammonia to the input, for example. (Scragg 2005, 51)
The fermentation process must be free from oxygen. Otherwise the presence of oxygen restricts the production of ethanol considerably. Stirring is usually needed to improve mass and heat transfer in a bio-reactor, especially in continuous reactors. (Scragg 2005 et a.l)
Feed conditions can vary from design specifications, which affect the performance of the distillation column, especially the location of a feed tray and the amount of stages needed for the separation. For example changes in upstream input and different process operating conditions can inhibit the profitability of distillation. (Tham 2011)
Incorrect reflux ratio can also impact on the result of distillation. If the reflux ratio is too small, an infinite number of trays are needed to reach the separation result. Moreover the efficiency of trays can decrease by fouling. Vapor flow conditions such as foaming, entrainment and flooding can also disturb the work of distillation. These phenomena can root from too small column diameter or incorrect pressure in the column. (Tham 2011)
Sterile conditions are also essential to maintain. The whole process can be contaminated if an unknown micro-organism enters the process. Therefore, the sterility of all process equipment needs to be ensured regularly. Electrical malfunctions and blockages in the pipes can also happen. (Micre 2011)
References
EPA (U.S. Environmental Protection Agency Region 7) 2007. Environmental Laws Applicable to Construction and Operate of Ethanol Plants. USA. Available at: http://www.epa.gov/region7/priorities/agriculture/pdf/ethanol_plants_manual.pdf
Liao, Wei and Saffron Chris 2008. Ethanol Production and Safety. Biosystems & Agricultural Engineering. Michigan, USA. [Cited 23 June 2011]. Available at: http://bioenergy.msu.edu/fuels/on_farm/on_farm_ethanol_production.pdf
Ministry of Environment 2011b. Bioetanolitehdas. [Internet pages]. [Cited 22 June 2011]. Available at: http://www.ymparisto.fi/default.asp?contentid=211826&lan=FI
MSDS Safety data for ethyl alcohol. 2011. [Internet pages]. [Cited 23 June 2011]. Available at: http://msds.chem.ox.ac.uk/ET/ethyl_alcohol.html
OSHA (Occupational Safety and Health Organization) (2005). U.S. Department of Labor. Available at: http://www.osha.gov/OshDoc/data_Hurricane_Facts/hydrogen_sulfide_fact.pdf
Nag, Ahindra 2007. Biofuels Refining and Performance. McGraw-Hill Professional Publishing. Ohio, USA. ISBN: 9780071594783. Available at: http://site.ebrary.com/lib/oulu/docDetail.action?docID=10210173&p00=handbook%20fermentation
Pöyry Environment 2006. Punkaharjun bioetanolitehdas, ympäristövaikutusten arviointiselostus. Suomen Bioetanoli Oy. Suomi. Available at: http://www.ymparisto.fi/download.asp?contentid=61456&lan=fi
Taavitsainen Toni 2006. Maatalouden biokaasulaitoksen perustaminen ja turvallisuustarkastelu. Savonia ammattikorkeakoulu (Malla2). ISBN: 952-203-041-4. Available at: http://portal.savonia.fi/img/amk/sisalto/teknologia_ja_ymparisto/ymparistotekniikka/Malla2Loppuraportti%281%29.pdf
|
