Diseño de tratamientos

TS fouling has been evaluated in detail (Arora et al. 2010; Challa et al. 2015; Wilkins et al. 2006) and several solutions have been proposed. The effect of oil (Singh et al. 2001), phytic acid (Tian et al. 2015), pH (Wilkins et al. 2006), mixed carbohydrates (Challa et al. 2015) and other components on fouling have been evaluated. One of the proposed and most discussed solutions to this costly issue is the addition of a filtration step on TS processing (Arora et al. 2010). Several applications of membrane systems were identified and useful in dewatering processing streams. Despite several advantages, membrane filtration has not acquired widespread use in the corn refinery and dry-grind corn ethanol plants. One of the major problems associated with membranes is flux decline by

pressure drop increase and membrane fouling. Accumulation of solute particles over the membrane surface causes the formation of gel layer or increasing

osmotic pressure at the membrane-solution interface, resulting in a permeate flux loss. Irreversible fouling is due to blocking of pores and solute adsorption, and causes the loss in membrane permeability which cannot be regained completely. Several models have been proposed to predict permeate flux and are classified into three main categories: osmotic pressure controlled, thin film, and resistance in series (Arora et al. 2010). Osmotically controlled models and thin film models are suitable for filtration of aqueous solutions of low and high molecular weight solutes, respectively, and they present limited application (Cornelissen et al. 2008).

The model regarding resistance in series is used in various research areas to understand flux decline behavior and membrane fouling. The latter model includes reversible and irreversible components of fouling, and it is an effective approach that predicts short and long-term flux decline (Arora et al. 2010). Lapišová et al. (2006) studied ceramic three-channel membranes in various separation arrangements to treat TS. Their results by coupling an ultrafiltration membrane with kDa pore size with a 0.2 µm pore size membrane provided reduction of 20% in dry matter and COD. The outcome of their study shows that the filtered TS can be recycled at a rate of 75% up to 40 cycles without

negatively affecting the ethanol production. According to their results, the composition of material had a great impact on membrane fouling. There is a significant lack of research in fouling behaviors, which limits the wide usage of membrane application in corn refineries (Lapišová et al. 2006).

Alkan-Ozkaynak et al. (2010) developed the first studies reported in utilizing chemical coagulation and flocculation as a treatment for TS, in order to reduce P levels. According to their study, the majority of P and solids in TS are in the dissolved form (<0.45 µm), which, if centrifuged, would yield a solid fraction with high crude protein and low P concentration. This solid fraction has similar

properties as WDG and can be directly utilized as the animal feed additive. On regards to the removal of colloidal inorganic and organic P, lime (CaCO3)

addition in TS achieved up to 85% of P removal. This removal was further evaluated using cationic and anionic flocculants, and it was found that anionic

polymers provided bigger and more stable flocs. The optimized results, of adding, 1.2 g of lime and 10 mg of polyacrylamide (anionic polymer) per liter of TS, would cost about $53,000 per million gallon of ethanol produced when their study was done (Alkan-Ozkaynak et al. 2010). Alkan-Ozkaynak and Karthikeyan (2011) further evaluated the performance of anaerobic digestion on lime-treated TS. They observed that treated-TS appears to be a superior substrate for AD, when compared to raw TS, resulting in the rapid production of high levels of biogas.

The separation of thin stillage into a solids fraction and the water is of potential industrial interest due to a number of factors, including enhancement in the anaerobic digestion and superior quality for water recycled into the system

(Alkan-Ozkaynak and Karthikeyan 2011), and the production of a co-product with higher bioavailability of amino and fatty acids (LoCascio and Dunbar 2014). The utilization of generally-regarded-as-safe (GRAS) chemical aids, e.g. GRAS anionic polymers, with a coupled dryer system generates a clarified TS and a dry feeding material in form of flocculated solids (LoCascio and Dunbar 2014).

2.3.2. Thermal processing

Combustion, gasification, and pyrolysis are three techniques that can be used to generate bioenergy from biomass feedstock. Biomass can be transformed into energy, primarily in the form of heat or electric power. Products produced from thermochemical conversion contain high concentrations of organic compounds, and thus are useful as concentrated sources of substrates for further utilization, i.e., conversion into fuels and chemicals (McKendry 2002).

Combustion occurs at a sufficient level of oxygen, and can result in flame

temperatures around 2000 °C. This process can be accomplished in a variety of equipment: grate-fired, suspension, fluidized bed combustors, furnaces and boilers, etc. (McKendry 2002). Gasification converts biomass into a flammable gas using an oxygen-deficient atmosphere, generally at temperatures between 750 and 850 °C (Heidenreich and Foscolo 2015). It can be accomplished by a variety of gasifiers, which most commonly have updraft, downdraft, or fluidized bed configurations. The synthesis gas, commonly known as syngas, produced is often rich in hydrogen and carbon monoxide, and can be combusted and used to drive electric generation equipment or power boilers. The residual byproduct of gasification is ash, which is often used as fertilizer. Pyrolysis is a process in which the feedstock is heated in the absence of oxygen, generally between temperatures of 200–500 °C (Branca and Di Blasi 2015). It results in bio-oil, char and gases. Little research has been done on thermal conversion of TS into valuable products.

Delgado et al. (2015) developed a platform to utilize hydrothermal carbonization, which is a system that operates at moderate temperatures (175–250 °C) and pressure conditions. This process produces a carbonized charcoal-like material with improved C:O ratio, along with a liquid fraction, that can be easily separated by filtration, according to their study. HTC also uses around 80% less energy to turn TS into a shelf stable product (Delgado et al. 2015). Wood et al. (2013) processed TS using HTC and indicated that dehydration was the principal

carbonization process occurring during the reaction conditions. The C:O ratio obtained at the derived hydrochar indicate that this material could be used as carbon–neutral fuels still having coal-like heating values (Wood et al. 2013). Despite producing chars with low surface area (2.2 m2 g−1 for center point TS), post-thermal treatments can increase surface area, potentially increasing the value of TS-derived hydrochar (Wood et al. 2013). Recent utilizations of

transformed hydrochars include supercapacitors for electrode materials (Falco et al. 2013), and sorbents for green-house gases, such as carbon dioxide (Sevilla et al. 2012).

A process developed by USDA researchers has established chemical, physical, and physicochemical methods for fractionating condensed fractions of TS

(Milczarek and Liu 2015), which leads to new coproducts and a new strategy for dewatering CDS. On regards to the fractionated CDS, all the new fractions showed faster drying rates than the CDS as control when convectively dried at 60 °C, except for the glycerol-rich fraction. To further demonstrate the improved drying performance of the new coproducts fractions, they used a drum dryer to dry a protein-rich fraction and the control. The results show that while both materials could be dried to a range of endpoint moisture contents, the dried protein-rich fraction exhibited a broader range of water activity and lighter color than CDS (Milczarek and Liu 2015).

Reports on thermal processing of TS demand that the carbonization could

a chemical absorbent and is a solid fuel with low ash and sulfur. Besides coproducts that could be directly applied as liquid fertilizer, other thermal processing techniques can yield better fractionation of TS. Dry TS could be readily turned into a shelf-stable, flaked product that could be marketed as a differentiated animal feed.